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" But with regard to the material world, we can at least go so 
far as this — we can perceive that events are brought about not by 
insulated interpositions of Divine power, exerted in each particular 
case, but by the establishment of general laws." 

W. Whewell : Bridgewater Treatise. 

" To conclude, therefore, let no man out of a weak conceit of 
sobriety, or an ill-applied moderation, think or maintain, that a 
man can search too far or be too well studied in the book of God's 
word, or in the book of God's works ; divinity or philosophy ; but 
rather let men endeavour an endless progress or proficience in both." 

Bacon : Advancement of Learning. 

Dow)i, Bromley, Kent, 

October 1st, 185 ( J. 











The right <>f Translation is n served. 



Introduction Page 1 


Variation under Domestication. 

Causes of Variability — Effects of Habit — Correlation of Growth — 
Inheritance — Character of Domestic Varieties — Difficulty of 
distinguishing between Varieties and Species — Origin of Domestic 
Varieties from one or more Species — Domestic Pigeons, their 
Differences and Origin — Principle of Selection anciently followed, 
its Effects — Methodical and Unconscious Selection — Unknown 
Origin of our Domestic Productions — Circumstances favourable 
to Man's power of Selection 7-43 


Variation under Nature. 

Variability — Individual differences — Doubtful species — Wide 
ranging, much diffused, and common species vary most — Spe- 
cies of the larger genera in any country vary more than the species 
of the smaller genera — Many of the species of the larger genera 
resemble varieties in being very closely, but unequally, related 
to each other, and in having restricted ranges . . . . 44-59 



Struggle foe Existence. 

Bears on natural selection — The term used in a wide sense — Geo- 
metrical powers of increase — Eapid increase of naturalised 
animals and plants — Nature of the checks to increase — Compe- 
tition universal — Effects of climate — Protection from the 
number of individuals — Complex relations of all animals and 
plants throughout nature — Struggle for life most severe between 
individuals and varieties of the same species ; often severe be- 
tween species of the same genus — The relation of organism to 
organism the most important of all relations . . Page 60-79 


Natural Selection. 

Natural Selection — its power compared with man's selection — its 
power on characters of trifling importance — its power at all ages 
and on both sexes — Sexual Selection — On the generality of inter- 
crosses between individuals of the same species — Circumstances 
favourable and unfavourable to Natural Selection, namely, 
intercrossing, isolation, number of individuals — Slow action — 
Extinction caused by Natural Selection — Divergence of Cha- 
racter, related to the diversity of inhabitants of any small area, 
and to naturalisation — Action of Natural Selection, through 
Divergence of Character and Extinction, on the descendants 
from a common parent — Explains the Grouping of all organic 
beings .. 80-130 


Laws of Variation. 

Effects of external conditions — Use and disuse, combined with 
natural selection ; organs of flight and of vision — Acclimatisa- 
tion — Correlation of growth — Compensation and economy of 
growth — False correlations — Multiple, rudimentary, and lowly 
organised structures variable — Parts developed in an unusual 
manner are highly variable : specific characters more variable 
than generic : secondary sexual characters variable — Species of 
the same genus vary in an analogous manner — Reversions to 
long-lost characters — Summary 131-170 



Difficulties on Theory. 

Difficulties on the theory of descent with modification — Transitions- 
Absence or rarity of transitional varieties — Transitions in habits 
of life — Diversified habits in the same species — Species with 
habits widely different from those of their allies — Organs of 
extreme perfection — Means of transition — Cases of difficulty — 
Natura non facit saltum — Organs of small importance — Organs 
not in all cases absolutely perfect — The law of Unity of Type 
and of the Conditions of Existence, embraced by the theory of 
Natural Selection Page 171-206 



Instincts comparable with habits, but different in their origin — 
Instincts graduated — Aphides and ants — Instincts variable — 
Domestic instincts, their origin — Natural instincts of the cuckoo, 
ostrich, and parasitic bees — Slave-making ants — Hive-bee, its 
cell-making instinct — Difficulties on the theory of the Natural 
Selection of instincts — Neuter or sterile insects — Summary 




Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close inter- 
breeding, removed by domestication — Laws governing the sterility 
of hybrids — Sterility not a special endowment, but incidental 
on other differences — Causes of the sterility of first crosses and 
of hybrids — Parallelism between the effects of changed con- 
ditions of life and crossing — Fertility of varieties when crossed 
and of their mongrel offspring not universal — Hybrids and 
mongrels compared independently of their fertility — Summary 




On the Imperfection of the Geological Record. 

On the absence of intermediate varieties at the present day — On 
the nature of extinct intermediate varieties ; on their number — 
On the vast lapse of time, as inferred from the rate of deposi- 
tion and of denudation — On the poorness of our palseontological 
collections — On the intermittence of geological formations — 
On the absence of intermediate varieties in any one formation 
— On the sudden appearance of groups of species — On their 
sudden appearance in the lowest known fossiliferous strata 

Page 279-311 


On the Geological Succession of Organic Beings. / 

On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their appear- 
ance and disappearance as do single species — On Extinction — 
On simultaneous changes in the forms of life throughout the 
world — On the affinities of extinct species to each other and to 
living species — On the state of development of ancient forms — 
On the succession of the same types within the same areas — 
Summary of preceding and present chapters . . . . 312-345 


Geographical Distribution. 

Present distribution cannot be accounted for by differences in phy- 
sical conditions — Importance of barriers — Affinity of the pro- 
ductions of the same continent — Centres of creation — Means 
of dispersal, by changes of climate and of the level of the land, 
and by occasional means — Dispersal during the Glacial period 
co-extensive with the world 346-382 


Geographical Distribution — con 

Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial Mam- 
mals — On the relation of the inhabitants of islands to those of 
the nearest mainland — On colonisation from the nearest source 
with subsequent modification — Summary of the last and pre- 
sent chapters Page 383-410 


Mutual Affinities of Organic Beings : Morphology : 
Embryology: Eudimentary Organs. 

Classification, groups subordinate to groups — Natural system — 
Rules and difficulties in classification, explained on the theory of 
descent with modification — Classification of varieties — Descent 
always used in classification — Analogical or adaptive characters 

— Affinities, general, complex and radiating — Extinction se- 
parates and defines groups — ■ Morphology, between members 
of the same class, between parts of the same individual — 
Embryology, laws of, explained by variations not supervening 
at an early age, and being inherited at a corresponding age 

— Rudimentary organs ; their origin explained — Summary 



Recapitulation and Conclusion. 

Recapitulation of the difficulties on the theory of Natural Selection 

— Recapitulation of the general and special circumstances in its 
favour — Causes of the general belief in the immutability of 
species — How far the theory of natural selection may be 
extended — Effects of its adoption on the study of Natural 
history — Concluding remarks 459-490 

Index 491 


The Diagram to front page 117, and to face the latter part of the 



When on board H.M.S. ' Beagle/ as naturalist, I was 
much struck with certain facts in the distribution of the 
inhabitants of South America, and in the geological rela- 
tions of the present to the past inhabitants of that con- 
tinent. These facts seemed to me to throw some light 
on the origin of species — that mystery of mysteries, as 
it has been called by one of our greatest philosophers. 
On my return home, it occurred to me, in 1837, that 
something might perhaps be made out on this question 
by patiently accumulating and reflecting on all sorts of 
facts which could possibly have any bearing on it. After 
five years' work I allowed myself to speculate on the 
subject, and drew up some short notes ; these I enlarged 
in 1844 into a sketch of the conclusions, which then 
seemed to me probable : from that period to the present 
day I have steadily pursued the same object. I hope 
that I may be excused for entering on these personal 
details, as I give them to show that I have not been hasty 
in coming to a decision. 

My work is now nearly finished ; but as it will take 
me two or three more years to complete it, and as my 
health is far from strong, I have been urged to publish 
this Abstract. I have more especially been induced 
to do this, as Mr. Wallace, who is now studying the 



natural history of the Malay archipelago, has arrived 
at almost exactly the same general conclusions that I 
have on the origin of species. Last year he sent to me 
a memoir on this subject, with a request that I would 
forward it to Sir Charles Lyell, who sent it to the Lin- 
nean Society, and it is published in the third volume of 
the Journal of that Society. Sir C. Lyell and Dr. 
Hooker, who both knew of my work — the latter having 
read my sketch of 1844 — honoured me by thinking it 
advisable to publish, with Mr. Wallace's excellent me- 
moir, some brief extracts from my manuscripts. 

This Abstract, which I now publish, must necessarily 
be imperfect. I cannot here give references and autho- 
rities for my several statements ; and I must trust to 
the reader reposing some confidence in my accuracy. 
No doubt errors will have crept in, though I hope I have 
always been cautious in trusting to good authorities 
alone. I can here give only the general conclusions at 
which I have arrived, with a few facts in illustration, 
but which, I hope, in most cases will suffice. No one 
can feel more sensible than I do of the necessity of here- 
after publishing in detail all the facts, with references, 
on which my conclusions have been grounded ; and I 
hope in a future work to do this. For I am well aware 
that scarcely a single point is discussed in this volume 
on which facts cannot be adduced, often apparently 
leading to conclusions directly opposite to those at which 
I have arrived. A fair result can be obtained only by 
fully stating and balancing the facts and arguments on 
both sides of each question ; and this cannot possibly be 
here done. 

I much regret that want of space prevents my having 
the satisfaction of acknowledging the generous assistance 
which I have received from very many naturalists, some 
of them personally unknown to me. I cannot, however, 


let this opportunity pass without expressing my deep 
obligations to Dr. Hooker, who for the last fifteen years 
has aided me in every possible way by his large stores 
of knowledge and his excellent judgment. 

In considering the Origin of Species, it is quite con- 
ceivable that a naturalist, reflecting on the mutual 
affinities of organic beings, on their embryological rela- 
tions, their geographical distribution, geological succes- 
sion, and other such facts, might come to the conclusion 
that each species had not been independently created, 
but had descended, like varieties, from other species. 
Nevertheless, such a conclusion, even if well founded, 
would be unsatisfactory, until it could be shown how 
the innumerable species inhabiting this world have been 
modified, so as to acquire that perfection of structure 
and coadaptation which most justly excites our admi- 
ration. Naturalists continually refer to external con- 
ditions, such as climate, food, &c, as the only possible 
cause of variation. In one very limited sense, as we 
shall hereafter see, this may be true ; but it is pre- 
posterous to attribute to mere external conditions, the 
structure, for instance, of the woodpecker, with its feet, 
tail, beak, and tongue, so admirably adapted to catch 
insects under the bark of trees. In the case of the 
misseltoe, which draws its nourishment from certain 
trees, which has seeds that must be transported by 
certain birds,. and which has flowers with separate sexes 
absolutely requiring the agency of certain insects to 
bring pollen from one flower to the other, it is equally 
preposterous to account for the structure of this parasite, 
with its relations to several distinct organic beings, by 
the effects of external conditions, or of habit, or of the 
volition of the plant itself. 

The author of the * Vestiges of Creation ' would, I 
presume, say that, after a certain unknown number of 



generations, some bird had given birth to a woodpecker, 
and some plant to the misseltoe, and that these had 
been produced perfect as we now see them; but this 
assumption seems to me to be no explanation, for it 
leaves the case of the coadaptations of organic beings to 
each other and to their physical conditions of life, un- 
touched and unexplained. 

It is, therefore, of the highest importance to gain a 
clear insight into the means of modification and co- 
adaptation. At the commencement of my observations 
it seemed to me probable that a careful study of domes- 
ticated animals and of cultivated plants would offer the 
best chance of making out this obscure problem. Nor 
have I been disappointed ; in this and in all other 
perplexing cases I have invariably found that our 
knowledge, imperfect though it be, of variation under 
domestication, afforded the best and safest clue. I may 
venture to express my conviction of the high value of 
such studies, although they have been very commonly 
neglected by naturalists. 

From these considerations, I shall devote the first 
chapter of this Abstract to Variation under Domestication. 
We shall thus see that a large amount of hereditary 
modification is at least possible ; and, what is equally or 
more important, we shall see how great is the power of 
man in accumulating by his Selection successive slight 
variations. I will then pass on to the variability of 
species in a state of nature ; but I shall, unfortunately, 
be compelled to treat this subject far too briefly, as it 
can be treated properly only by giving long catalogues 
of facts. We shall, however, be enabled to discuss 
what circumstances are most favourable to variation. 
In the next chapter the Struggle for Existence amongst 
all organic beings throughout the world, which inevi- 
tably follows from their high geometrical powers of 


increase, will be treated of. This is the doctrine of 
Malthus, applied to the whole animal and vegetable 
kingdoms. As many more individuals of each species 
are born than can possibly survive ; and as, conse- 
quently, there is a frequently recurring struggle for 
existence, it follows that any being, if it vary however 
slightly in any manner profitable to itself, under the 
complex and sometimes varying conditions of life, will 
have a better chance of surviving, and thus be naturally 
selected. From the strong principle of inheritance, any 
selected variety will tend to propagate its new and 
modified form. 

This fundamental subject of Natural Selection will 
be treated at some length in the fourth chapter ; and we 
shall then see how Natural Selection almost inevitably 
causes much Extinction of the less improved forms of 
life, and induces what I have called Divergence of 
Character. In the next chapter I shall discuss the 
complex and little known laws of variation and of corre- 
lation of growth. In the four succeeding chapters, the 
most apparent and gravest difficulties on the theory 
will be given : namely, first, the difficulties of transi- 
tions, or in understanding how a simple being or a simple 
organ can be changed and perfected into a highly 
developed being or elaborately constructed organ ; 
secondly, the subject of Instinct, or the mental powers of 
animals ; thirdly, Hybridism, or the infertility of species 
and the fertility of varieties when intercrossed ; and 
fourthly, the imperfection of the Geological Kecord. 
In the next chapter I shall consider the geological 
succession of organic beings throughout time ; in the 
eleventh and twelfth, their geographical distribution 
throughout space ; in the thirteenth, their classification 
or mutual affinities, both when mature and in an em- 
bryonic condition. In the last chapter I shall give a 


brief recapitulation of the whole work, and a few con- 
cluding remarks. 

No one ought to feel surprise at much remaining as 
yet unexplained in regard to the origin of species and 
varieties, if he makes due allowance for our profound 
ignorance in regard to the mutual relations of all the 
beings which live around us. Who can explain why one 
species ranges widely and is very numerous, and why 
another allied species has a narrow range and is rare ? 
Yet these relations are of the highest importance, for 
they determine the present welfare, and, as I believe, 
the future success and modification of every inhabitant 
of this world. Still less do we know of the mutual 
relations of the innumerable inhabitants of the world 
during the many past geological epochs in its history. 
Although much remains obscure, and will long remain 
obscure, I can entertain no doubt, after the most de- 
liberate study and dispassionate judgment of which I 
am capable, that the view which most naturalists enter- 
tain, and which I formerly entertained — namely, that 
each species has been independently created — is erro- 
neous. I am fully convinced that species are not im- 
mutable ; but that those belonging to what are called 
the same genera are lineal descendants of some other 
and generally extinct species, in the same manner as 
the acknowledged varieties of any one species are the 
descendants of that species. Furthermore, I am con- 
vinced that Natural Selection has been the main but 
not exclusive means of modification. 



Variation under Domestication. 

Causes of Variability — Effects of Habit — Correlation of Growth — 
Inheritance — Character of Domestic Varieties — Difficulty of 
distinguishing between Varieties and Species — Origin of Domestic 
Varieties from one or more Species — Domestic Pigeons, their 
Differences and Origin — Principle of Selection anciently followed, 
its Effects — Methodical and Unconscious Selection — Unknown 
Origin of our Domestic Productions — Circumstances favourable 
to Man's power of Selection. 

When we look to the individuals of the same variety or 
sub-variety of our older cultivated plants and animals, 
one of the first points which strikes us, is, that they 
generally differ much more from each other, than do the 
individuals of any one species or variety in a state of 
nature. When we reflect on the vast diversity of the 
plants and animals which have been cultivated, and 
which have varied during all ages under the most 
different climates and treatment, I think we are driven 
to conclude that this greater variability is simply due to 
our domestic productions having been raised under con- 
ditions of life not so uniform as, and somewhat different 
from, those to which the parent-species have been exposed 
under nature. There is, also, I think, some probability 
in the view propounded by Andrew Knight, that this 
variability may be partly connected with excess of food. 
It seems pretty clear that organic beings must be ex- 
posed during several generations to the new conditions 
of life to cause any appreciable amount of variation ; 
and that when the organisation has once begun to vary, 
it generally continues to vary for many generations. 


No case is on record of a variable being ceasing to be 
variable under cultivation. Our oldest cultivated plants, 
such as wheat, still often yield new varieties : our oldest 
domesticated animals are still capable of rapid improve- 
ment or modification. 

It has been disputed at what period of life the causes 
of variability, whatever they may be, generally act ; 
whether during the early or late period of development 
of the embryo, or at the instant of conception. Greoffroy 
St. Hilaire's experiments show that unnatural treatment 
of the embryo causes monstrosities; and monstrosities 
cannot be separated by any clear line of distinction 
from mere variations. But I am strongly inclined to 
suspect that the most frequent cause of variability may 
be attributed to the male and female reproductive 
elements having been affected prior to the act of con- 
ception. Several reasons make me believe in this ; but 
the chief one is the remarkable effect which confine- 
ment or cultivation has on the functions of the repro- 
ductive system ; this system appearing to be far more 
susceptible than any other part of the organisation, to 
the action of any change in the conditions of life. No- 
thing is more easy than to tame an animal, and few 
things more difficult than to get it to breed freely under 
confinement, even in the many cases when the male and 
female unite. How many animals there are which will 
not breed, though living long under not very close con- 
finement in their native country! This is generally 
attributed to vitiated instincts ; but how many cultivated 
plants display the utmost vigour, and yet rarely or never 
seed ! In some few such cases it has been found out 
that very trifling changes, such as a little more or less 
water at some particular period of growth, will determine 
whether or not the plant sets a seed. I cannot here 
enter on the copious details which I have collected on 


this curious subject ; but to show how singular the laws 
are which determine the reproduction of animals under 
confinement, I may just mention that carnivorous animals, 
even from the tropics, breed in this country pretty 
freely under confinement, with the exception of the 
plantigrades or bear family ; whereas, carnivorous birds, 
with the rarest exceptions, hardly ever lay fertile eggs. 
Many exotic plants have pollen utterly worthless, in the 
same exact condition as in the most sterile hybrids. 
When, on the one hand, we see domesticated animals 
and plants, though often weak and sickly, yet breeding 
quite freely under confinement ; and when, on the other 
hand, we see individuals, though taken young from a 
state of nature, perfectly tamed, long-lived, and healthy 
(of which I could give numerous instances), yet having 
their reproductive system so seriously affected by un- 
perceived causes as to fail in acting, we need not be 
surprised at this system, when it does act under con- 
finement, acting not quite regularly, and producing off- 
spring not perfectly like their parents or variable. 

Sterility has been said to be the bane of horticulture ; 
but on this view we owe variability to the same cause 
which produces sterility ; and variability is the source of 
all the choicest productions of the garden. I may add, 
that as some organisms will breed most freely under 
the most unnatural conditions (for instance, the rabbit 
and ferret kept in hutches), showing that their repro- 
ductive system has not been thus affected ; so will some 
animals and plants withstand domestication or cultiva- 
tion, and vary very slightly — perhaps hardly more than 
in a state of nature. 

A long list could easily be given of " sporting plants;" 
by this term gardeners mean a single bud or offset, 
which suddenly assumes a new and sometimes very dif- 
ferent character from that of the rest of the plant. 


10 VAKIATION Chap. I. 

Such buds can be propagated by grafting, &c, and 
sometimes by seed. These "sports" are extremely 
rare under nature, but far from rare under cultivation ; 
and in this case we see that the treatment of the parent 
has affected a bud or offset, and not the ovules or pollen. 
But it is the opinion of most physiologists that there is no 
essential difference between a bud and an ovule in their 
earliest stages of formation; so that, in fact, "sports" 
support my view, that variability may be largely attri- 
buted to the ovules or pollen, or to both, having been 
affected by the treatment of the parent prior to the act of 
conception. These cases anyhow show that variation is 
not necessarily connected, as some authors have sup- 
posed, with the act of generation. 

Seedlings from the same fruit, and the young of the 
same litter, sometimes differ considerably from each 
other, though both the young and the parents, as Muller 
has remarked, have apparently been exposed to exactly 
the same conditions of life ; and this shows how unim- 
portant the direct effects of the conditions of life are in 
comparison with the laws of reproduction, and of growth, 
and of inheritance ; for had the action of the conditions 
been direct, if any of the young had varied, all would 
probably have varied in the same manner. To judge how 
much, in the case of any variation, we should attribute 
to the direct action of heat, moisture, light, food, &c, 
is most difficult : my impression is, that with animals 
such agencies have produced very little direct effect, 
though apparently more in the case of plants. Under 
this point of view, Mr. Buckman's recent experiments 
on plants seem extremely valuable. When all or nearly 
all the individuals exposed to certain conditions are 
affected in the same way, the change at first appears to 
be directly due to such conditions ; but in some cases it 
can be shown that quite opposite conditions produce 


similar changes of structure. Nevertheless some slight 
amount of change may, I think, be attributed to. the 
direct action of the conditions of life — as, in some cases, 
increased size from amount of food, colour from par- 
ticular kinds of food and from light, and perhaps the 
thickness of fur from climate. 

Habit also has a decided influence, as in the period 
of flowering with plants when transported from one 
climate to another. In animals it has a more marked 
effect ; for instance, I find in the domestic duck that the 
bones of the wing weigh less and the bones of the leg 
more, in proportion to the whole skeleton, than do the 
same bones in the wild-duck ; and I presume that this 
change may be safely attributed to the domestic duck 
flying much less, and walking more, than its wild parent. 
The great and inherited development of the udders in 
cows and goats in countries where they are habitually 
milked, in comparison with the state of these organs 
in other countries, is another instance of the effect of 
use. Not a single domestic animal can be named 
which has not in some country drooping ears ; and the 
view suggested by some authors, that the drooping is due 
to the disuse of the muscles of the ear, from the animals 
not being much alarmed by danger, seems probable. 

There are many laws regulating variation, some few 
of which can be dimly seen, and will be hereafter briefly 
mentioned. I will here only allude to what may be 
called correlation of growth. Any change in the embryo 
or larva will almost certainly entail changes in the 
mature animal. In monstrosities, the correlations be- 
tween quite distinct parts are very curious ; and many 
instances are given in Isidore Geoffroy St. Hilaire's great 
work on this subject. Breeders believe that long limbs 
are almost always accompanied by an elongated head. 
Some instances of correlation are quite whimsical : thus 

12 VARIATION Chap. I. 

cats with blue eyes are invariably deaf ; colour and con- 
stitutional peculiarities go together, of which many 
remarkable cases could be given amongst animals and 
plants. From the facts collected by Heusinger, it ap- 
pears that white sheep and pigs are differently affected 
from coloured individuals by certain vegetable poisons. 
Hairless dogs have imperfect teeth; long-haired and 
coarse-haired animals are apt to have, as is asserted, long 
or many horns ; pigeons with feathered feet have skin 
between their outer toes ; pigeons with short beaks have 
small feet, and those with long beaks large feet. Hence, 
if man goes on selecting, and thus augmenting, any pe- 
culiarity, he will almost certainly unconsciously modify 
other parts of the structure, owing to the mysterious 
laws of the correlation of growth. 

The result of the various, quite unknown, or dimly 
seen laws of variation is infinitely complex and diversified. 
It is well worth while carefully to study the several 
treatises published on some of our old cultivated plants, 
as on the hyacinth, potato, even the dahlia, &c. ; and it 
is really surprising to note the endless points in struc- 
ture and constitution in which the varieties and sub- 
varieties differ slightly from each other. The whole 
organisation seems to have become plastic, and tends to 
depart in some small degree from that of the parental 

Any variation which is not inherited is unimportant 
for us. But the number and diversity of inheritable 
deviations of structure, both those of slight and those of 
considerable physiological importance, is endless. Dr. 
Prosper Lucas's treatise, in two large volumes, is the 
fullest and the best on this subject. No breeder doubts 
how strong is the tendency to inheritance : like produces 
like is his fundamental belief: doubts have been thrown 
on this principle by theoretical writers alone. When a 


deviation appears not unfrequently, and we see it in the 
father and child, we cannot tell whether it may not be 
due to the same original cause acting on both ; but when 
amongst individuals, apparently exposed to the same 
conditions, any very rare deviation, due to some extraor- 
dinary combination of circumstances, appears in the 
parent — say, once amongst several million individuals — 
and it reappears in the child, the mere doctrine of 
chances almost compels us to attribute its reappearance 
to inheritance. Every one must have heard of cases of 
albinism, prickly skin, hairy bodies, &c, appearing in 
several members of the same family. If strange and 
rare deviations of structure are truly inherited, less 
strange and commoner deviations may be freely ad- 
mitted to be inheritable. Perhaps the correct way of" 
viewing the whole subject, would be, to look at the in- 
heritance of every character whatever as the rule, and 
non-inheritance as the anomaly. 

The laws governing inheritance are quite unknown ; 
no one can say why the same peculiarity in different 
individuals of the same species, and in individuals of 
different species, is sometimes inherited and sometimes 
not so ; why the child often reverts in certain characters 
to its grandfather or grandmother or other much more 
remote ancestor ; why a peculiarity is often transmitted 
from one sex to both sexes, or to one sex alone, more 
commonly but not exclusively to the like sex. It is 
a fact of some little importance to us, that peculi- 
arities appearing in the males of our domestic breeds 
are often transmitted either exclusively, or in a much 
greater degree, to males alone. A much more im- 
portant rule, which I think may be trusted, is that, at 
whatever period of life a peculiarity first appears, it 
tends to appear in the offspring at a corresponding age, 
though sometimes earlier. In many cases this could 

14 VAKIATION Chap. I. 

not be otherwise : thus the inherited peculiarities in the 
horns of cattle could appear only in the offspring when 
nearly mature ; peculiarities in the silkworm are known 
to appear at the corresponding caterpillar or cocoon 
stage. But hereditary diseases and some other facts 
make me believe that the rule has a wider extension, 
and that when there is no apparent reason why a pecu- 
liarity should appear at any particular age, yet that it 
does tend to appear in the offspring at the same period 
at which it first appeared in the parent. I believe tins 
rule to be of the highest importance in explaining the 
laws of embryology. These remarks are of course con- 
fined to the first appearance of the peculiarity, and not 
to its primary cause, which may have acted on the 
ovules or male element ; in nearly the same manner as 
in the crossed offspring from a short-horned cow by a 
long-horned bull, the greater length of horn, though 
appearing late in life, is clearly due to the male 

Having alluded to the subject of reversion, I may 
here refer to a statement often made by naturalists — 
namely, that our domestic varieties, when run wild, 
gradually but certainly revert in character to their 
aboriginal stocks. Hence it has been argued that no 
deductions can be drawn from domestic races to species 
in a state of nature. I have in vain endeavoured to 
discover on what decisive facts the above statement has 
so often and so boldly been made. There would be 
great difficulty in proving its truth : we may safely con- 
clude that very many of the most strongly-marked 
domestic varieties could not possibly live in a wild state. 
In many cases we do not know what the aboriginal stock 
was, and so could not tell whether or not nearly perfect 
reversion had ensued. It would be quite necessary, in 
order to prevent the effects of intercrossing, that only a 


single variety should be turned loose in its new home. 
Nevertheless, as our varieties certainly do occasionally 
revert in some of their characters to ancestral forms, it 
seems to me not improbable, that if we could succeed in 
naturalising, or were to cultivate, during many genera- 
tions, the several races, for instance, of the cabbage, in 
very poor soil (in which case, however, some effect would 
have to be attributed to the direct action of the poor 
soil), that they would to a large extent, or even wholly, 
revert to the wild aboriginal stock. Whether or not the 
experiment would succeed, is not of great importance 
for our line of argument ; for by the experiment itself 
the conditions of life are changed. If it could be shown 
that our domestic varieties manifested a strong tendency 
to reversion, — that is, to lose their acquired characters, 
whilst kept under unchanged conditions, and whilst kept 
in a considerable body, so that free intercrossing might 
check, by blending together, any slight deviations of 
structure, in such case, I grant that we could deduce 
nothing from domestic varieties in regard to species. 
But there is not a shadow of evidence in favour of this 
view: to assert that we could not breed our cart and 
race-horses, long and short-horned cattle, and poultry 
of various breeds, and esculent vegetables, for an almost 
infinite number of generations, would be opposed to all 
experience. I may add, that when under nature the 
conditions of life do change, variations and reversions 
of character probably do occur ; but natural selection, 
as will hereafter be explained, will determine how far 
the new characters thus arising shall be preserved. 

When we look to the hereditary varieties or races of 
our domestic animals and plants, and compare them with 
species closely allied together, we generally perceive in 
each domestic race, as already remarked, less uniformity 
of character than in true species. Domestic races of 

16 VAKIATION Chap. I. 

the same species, also, often have a somewhat monstrous 
character; by which I mean, that, although differing 
from each other, and from the other species of the same 
genus, in several trifling respects, they often differ in an 
extreme degree in some one part, both when compared 
one with another, and more especially when compared 
with all the species in nature to which they are nearest 
allied. With these exceptions (and with that of the 
perfect fertility of varieties when crossed, — a subject 
hereafter to be discussed), domestic races of the same 
species differ from each other in the same manner as, 
only in most cases in a lesser degree than, do closely- 
allied species of the same genus in a state of nature. I 
think this must be admitted, when we find that there are 
hardly any domestic races, either amongst animals or 
plants, which have not been ranked by some competent 
judges as mere varieties, and by other competent judges 
as the descendants of aboriginally distinct species. If 
any marked distinction existed between domestic races 
and species, this source of doubt could not so perpetu- 
ally recur. It has often been stated that domestic races 
do not differ from each other in characters of generic 
value. I think it could be shown that this statement is 
hardly correct; but naturalists differ most widely in 
determining what characters are of generic value ; all 
such valuations being at present empirical. Moreover, 
on the view of the origin of genera which I shall pre- 
sently give, we have no right to expect often to meet 
with generic differences in our domesticated productions. 
When we attempt to estimate the amount of structural 
difference between the domestic races of the same species, 
we are soon involved in doubt, from not knowing whether 
they have descended from one or several parent-species. 
This point, if it could be cleared up, would be interest- 
ing ; if, for instance, it could be shown that the grey- 


hound, bloodhound, terrier, spaniel, and bull-dog, which 
we all know propagate their kind so truly, were the off- 
spring of any single species, then such facts would have 
great weight in making us doubt about the immutability 
of the many very closely allied and natural species — for 
instance, of the many foxes — inhabiting different quarters 
of the world. I do not believe, as we shall presently 
see, that all our dogs have descended from any one 
wild species ; but, in the case of some other domestic 
races, there is presumptive, or even strong, evidence in 
favour of this view. 

It has often been assumed that man has chosen for 
domestication animals and plants having an extra- 
ordinary inherent tendency to vary, and likewise to 
withstand diverse climates. I do not dispute that these 
capacities have added largely to the value of most of 
our domesticated productions ; but how could a savage 
possibly know, when he first tamed an animal, whether 
it would vary in succeeding generations, and whether it 
would endure other climates ? Has the little variability 
of the ass or guinea-fowl, or the small power of endurance 
of warmth by the rein-deer, or of cold by the common 
camel, prevented their domestication? I cannot doubt 
that if other animals and plants, equal in number to 
our domesticated productions, and belonging to equally 
diverse classes and countries, were taken from a state 
of nature, and could be made to breed for an equal 
number of generations under domestication, they would 
vary on an average as largely as the parent species of 
our existing domesticated productions have varied. 

In the case of most of our anciently domesticated 
animals and plants, I do not think it is possible to come 
to any definite conclusion, whether they have descended 
from one or several species. The argument mainly 
relied on by those who believe in the multiple origin 

18 VARIATION Chap. I. 

of our domestic animals is, that we find in the most 
ancient records, more especially on the monuments of 
Egypt, much diversity in the breeds ; and that some of 
the breeds closely resemble, perhaps are identical with, 
those still existing. Even if this latter fact were 
found more strictly and generally true than seems 
to me to be the case, what does it show, but that some 
of our breeds originated there, four or five thousand 
years ago? But Mr. Horner's researches have ren- 
dered it in some degree probable that man sufficiently 
civilized to have manufactured pottery existed in the 
valley of the Nile thirteen or fourteen thousand years ago ; 
and who will pretend to say how long before these ancient 
periods, savages, like those of Tierra del Fuego or Aus- 
tralia, who possess a semi-domestic dog, may not have 
existed in Egypt ? 

The whole subject must, I think, remain vague ; 
neverthelsss, I may, without here entering on any 
details, state that, from geographical and other con- 
siderations, I think it highly probable that our domestic 
dogs have descended from several wild species. In 
regard to sheep and goats I can form no opinion. I 
should think, from facts communicated to me by Mr. 
Blyth, on the habits, voice, and constitution, &c, of the 
humped Indian cattle, that these had descended from 
a different aboriginal stock from our European cattle ; 
and several competent judges believe that these latter 
have had more than one wild parent. With respect to 
horses, from reasons which I cannot give here, I am 
doubtfully inclined to believe, in opposition to several 
authors, that all the races have descended from one 
wild stock. Mr. Blyth, whose opinion, from his large 
and varied stores of knowledge. I should value more 
than that of almost any one, thinks that all the breeds 
of poultry have proceeded from the common wild 


Indian fowl (Gallus bankiva). In regard to ducks and 
rabbits, the breeds of which differ considerably from 
each other in structure, I do not doubt that they all 
have descended from the common wild duck and rabbit. 
The doctrine of the origin of our several domestic 
races from several aboriginal stocks, has been carried to 
an absurd extreme by some authors. They believe that 
every race which breeds true, let the distinctive cha- 
racters be ever so slight, has had its wild prototype. 
At this rate there must have existed at least a score of 
species of wild cattle, as many sheep, and several goats 
in Europe alone, and several even within Great Britain. 
One author believes that there formerly existed in 
Great Britain eleven wild species of sheep peculiar to it ! 
When we bear in mind that Britain has now hardly one 
peculiar mammal, and France but few distinct from those 
of Germany and conversely, and so with Hungary, 
Spain, &c, but that each of these kingdoms possesses 
several peculiar breeds of cattle, sheep, &c, we must 
admit that many domestic breeds have originated in 
Europe ; for whence could they have been derived, as 
these several countries do not possess a number of 
peculiar species as distinct parent-stocks? So it is in 
India, Even in the case of the domestic dogs of the 
whole world, which I fully admit have probably de- 
scended from several wild species, I cannot doubt that 
there has been an immense amount of inherited varia- 
tion. Who can believe that animals closely resembling 
the Italian greyhound, the bloodhound, the bull-dog, 
or Blenheim spaniel, &c. — so unlike all wild Canidse 
— ever existed freely in a state of nature ? It has 
often been loosely said that all our races of dogs have 
been produced by the crossing of a few aboriginal 
species ; but by crossing we can get only forms in some 
degree intermediate between their parents ; and if we 


account for our several domestic races by this process, 
we must admit the former existence of the most extreme 
forms, as the Italian greyhound, bloodhound, bull-dog, 
&c, in the wild state. Moreover, the possibility of 
making distinct races by crossing has been greatly ex- 
aggerated. There can be no doubt that a race may be 
modified by occasional crosses, if aided by the careful 
selection of those individual mongrels, which present any 
desired character ; but that a race could be obtained 
nearly intermediate between two extremely different 
races or speceies, I can hardly believe. Sir J. Sebright 
expressly experimentised for this object, and failed. The 
offspring from the first cross between two pure breeds 
is tolerably and sometimes (as I have found with 
pigeons) extremely uniform, and everything seems simple 
enough ; but when these mongrels are crossed one with 
another for several generations, hardly two of them will 
be alike, and then the extreme difficulty, or rather utter 
hopelessness, of the task becomes apparent. Certainly, 
a breed intermediate between two very distinct breeds 
could not be got without extreme care and long-con- 
tinued selection ; nor can I find a single case on record 
of a permanent race having been thus formed. 

On the Breeds of the Domestic Pigeon. — Believing 
that it is always best to study some special group, I 
have, after deliberation, taken up domestic pigeons. 
I have kept every breed which I could purchase or 
obtain, and have been most kindly favoured with 
skins from several quarters of the world, more espe- 
cially by the Hon. W. Elliot from India, and by the 
Hon. C. Murray from Persia. Many treatises in dif- 
ferent languages have been published on pigeons, and 
some of them are very important, as being of con- 
siderable antiquity. I have associated with several 
eminent fanciers, and have been permitted to join two 


of the London Pigeon Clubs. The diversity of the 
breeds is something astonishing. Compare the English 
carrier and the short-faced tumbler, and see the won- 
derful difference in their beaks, entailing corresponding 
differences in their skulls. The carrier, more especially 
the male bird, is also remarkable from the wonderful 
development of the carunculated skin about the head, 
and this is accompanied by greatly elongated eyelids, 
very large external orifices to the nostrils, and a wide 
gape of mouth. The short-faced tumbler has a beak 
in outline almost like that of a finch ; and the common 
tumbler has the singular and strictly inherited habit of 
flying at a great height in a compact flock, and tumbling 
in the air head over heels. The runt is a bird of great 
size, with long, massive beak and large feet; some of 
the sub-breeds of runts have very long necks, others very 
long wings and tails, others singularly short tails. The 
barb is allied to the carrier, but, instead of a very long 
beak, has a very short and very broad one. The pouter 
has a much elongated body, wings, and legs; and its 
enormously developed crop, which it glories in inflating, 
may well excite astonishment and even laughter. The 
turbit has a very short and conical beak, with a line of 
reversed feathers down the breast ; and it has the habit 
of continually expanding slightly the upper part of the 
oesophagus. The Jacobin has the feathers so much 
reversed along the back of the neck that they form 
a hood, and it has, proportionally to its size, much 
elongated wing and tail feathers. The trumpeter and 
laugher, as their names express, utter a very different 
coo from the other breeds. The fantail has thirty or 
even forty tail-feathers, instead of twelve or fourteen, 
the normal number in all members of the great pigeon 
family ; and these feathers are kept expanded, and are 
carried so erect that in good birds the head and tail 


touch; the oil-gland is quite aborted. Several other 
less distinct breeds might have been specified. 

In the skeletons of the several breeds, the develop- 
ment of the bones of the face in length and breadth and 
curvature differs enormously. The shape, as well as 
the breadth and length of the ramus of the lower jaw, 
varies in a highly remarkable manner. The number of 
the caudal and sacral vertebrae vary ; as does the number 
of the ribs, together with their relative breadth and the 
presence of processes. The size and shape of the 
apertures in the sternum are highly variable ; so is the 
degree of divergence and relative size of the two arms of 
the furcula. The proportional width of the gape of mouth, 
the proportional length of the eyelids, of the orifice of 
the nostrils, of the tongue (not always in strict correla- 
tion witli the length of beak), the size of the crop and 
of the upper part of the oesophagus ; the development 
and abortion of the oil-gland ; the number of the primary 
wing and caudal feathers ; the relative length of wing 
and tail to each other and to the body ; the relative 
length of leg and of the feet ; the number of scutellae 
on the toes, the development of skin between the toes, 
are all points of structure which are variable. The period 
at which the perfect plumage is acquired varies, as does 
the state of the down with which the nestling birds are 
clothed when hatched. The shape and size of the eggs 
vary. The manner of flight differs remarkably ; as does 
in some breeds the voice and disposition. Lastly, in 
certain breeds, the males and females have come to 
differ to a slight degree from each other. 

Altogether at least a score of pigeons might be 
chosen, which if shown to an ornithologist, and he were 
told that they were wild birds, would certainly, I think, 
be ranked by him as well-defined species. Moreover, 
I do not believe that any ornithologist would place 


the English carrier, the short-faced tumbler, the runt, 
the barb, pouter, and fantail in the same genus ; more 
especially as in each of these breeds several truly- 
inherited sub-breeds, or species as he might have called 
them, could be shown him. 

Great as the differences are between the breeds of 
pigeons, I am fully convinced that the common opinion of 
naturalists is correct, namely, that all have descended 
from the. rock-pigeon (Columba livia), including under 
this term several geographical races or sub-species, which 
differ from each other in the most trifling respects. As 
several of the reasons which have led me to this belief 
are in some degree applicable in other cases, I will here 
briefly give them. If the several breeds are not varieties, 
and have not proceeded from the rock-pigeon, they must 
have descended from at least seven or eight aboriginal 
stocks ; for it is impossible to make the present domestic 
breeds by the crossing of any lesser number : how, for 
instance, could a pouter be produced by crossing two 
breeds unless one of the parent-stocks possessed the 
characteristic enormous crop ? The supposed aboriginal 
stocks must all have been rock-pigeons, that is, not 
breeding or willingly perching on trees. But besides 
C. livia, with its geographical sub-species, only two or 
three other species of rock-pigeons are known ; and these 
have not any of the characters of the domestic breeds. 
Hence the supposed aboriginal stocks must either still 
exist in the countries where they were originally domes- 
ticated, and yet be unknown to ornithologists ; and this, 
considering their size, habits, and remarkable characters, 
seems very improbable ; or they must have become 
extinct in the wild state. But birds breeding on preci- 
pices, and good fliers, are unlikely to be exterminated ; 
and the common rock-pigeon, which has the same habits 
with the domestic breeds, has not been exterminated 


even on several of the smaller British islets, or on the 
shores of the Mediterranean. Hence the supposed ex- 
termination of so many species having similar habits 
with the rock-pigeon seems to me a very rash assump- 
tion. Moreover, the several above-named domesticated 
breeds have been transported to all parts of the world, 
and, therefore, some of them must have been carried 
back again into their native country ; but not one has 
ever become wild or feral, though the dovecot-pigeon, 
which is the rock-pigeon in a very slightly altered state, 
has become feral in several places. Again, all recent 
experience shows that it is most difficult to get any wild 
animal to breed freely under domestication ; yet on the 
hypothesis of the multiple origin of our pigeons, it must 
be assumed that at least seven or eight species were so 
thoroughly domesticated in ancient times by half-civi- 
lized man, as to be quite prolific under confinement. 

An argument, as it seems to me, of great weight, and 
applicable in several other cases, is, that the above- 
specified breeds, though agreeing generally in constitu- 
tion, habits, voice, colouring, and in most parts of their 
structure, with the wild rock-pigeon, yet are certainly 
highly abnormal in other parts of their structure : we 
may look in vain throughout the whole great family of 
Columbidae for a beak like that of the English carrier, 
or that of the short-faced tumbler, or barb ; for reversed 
feathers like those of the jacobin ; for a crop like that 
of the pouter ; for tail-feathers like those of the fan tail. 
Hence it must be assumed not only that half-civilized 
man succeeded in thoroughly domesticating several 
species, but that he intentionally or by chance picked 
out extraordinarily abnormal species ; and further, that 
these very species have since all become extinct or un- 
known. So many strange contingencies seem to me 
improbable in the highest degree. 


Some facts in regard to the colouring of pigeons 
well deserve consideration. The rock-pigeon is of a 
slaty-blue, and has a white rump (the Indian sub- 
species, C. intermedia of Strickland, having it bluish) ; 
the tail has a terminal dark bar, with the bases of the 
outer feathers externally edged with white ; the wings 
have two black bars; some semi-domestic breeds and 
some apparently truly wild breeds have, besides the 
two black bars, the wings chequered with black. These 
several marks do not occur together in any other species 
of the whole family. Now, in every one of the domestic 
breeds, taking thoroughly well-bred birds, all the above 
marks, even to the white edging of the outer tail- 
feathers, sometimes concur perfectly developed. More- 
over, when two birds belonging to two distinct breeds 
are crossed, neither of which is blue or has any of the 
above-specified marks, the mongrel offspring are very 
apt suddenly to acquire these characters ; for instance, 
I crossed some uniformly white fantails with some 
uniformly black barbs, and they produced mottled 
brown and black birds ; these I again crossed together, 
and one grandchild of the pure white fantail and pure 
black barb was of as beautiful a blue colour, with the 
white rump, double black wing-bar, and barred and 
white-edged tail-feathers, as any wild rock-pigeon ! We 
can understand these facts, on the well-known principle 
of reversion to ancestral characters, if all the domestic 
breeds have descended from the rock-pigeon. But if 
we deny this, we must make one of the two following 
highly improbable suppositions. Either, firstly, that all 
the several imagined aboriginal stocks were coloured 
and marked like the rock-pigeon, although no other 
existing species is thus coloured and marked, so that in 
each separate breed there might be a tendency to revert 
to the very same colours and markings. Or, secondly, 



that each breed, even the purest, has within a dozen or, 
at most, within a score of generations, been crossed by 
the rock-pigeon : I say within a dozen or twenty genera- 
tions, for we know of no fact countenancing the belief 
that the child ever reverts to some one ancestor, removed 
by a greater number of generations. In a breed which 
has been crossed only once with some distinct breed, the 
tendency to reversion to any character derived from such 
cross will naturally become less and less, as in each suc- 
ceeding generation there will be less of the foreign blood ; 
but when there has been no cross with a distinct breed, 
and there is a tendency in both parents to revert to a 
character, which has been lost during some former gene- 
ration, this tendency, for all that we can see to the 
contrary, may be transmitted undiminished for an indefi- 
nite number of generations. These two distinct cases are 
often confounded in treatises on inheritance. 

Lastly, the hybrids or mongrels from between all the 
domestic breeds of pigeons are perfectly fertile. I can 
state this from my own observations, purposely made 
on the most distinct breeds. Now, it is difficult, per- 
haps impossible, to bring forward one case of the hybrid 
offspring of two animals clearly distinct being themselves 
perfectly fertile. Some authors believe that long-con- 
tinued domestication eliminates this strong tendency to 
sterility : from the history of the dog I think there is 
some probability in this hypothesis, if applied to species 
closely related together, though it is unsupported by a 
single experiment. But to extend the hypothesis so 
far as to suppose that species, aboriginally as distinct 
as carriers, tumblers, pouters, and fantails now are, 
should yield offspring perfectly fertile, inter se, seems to 
me rash in the extreme. 

From these several reasons, namely, the improbability 
of man having formerly got seven or eight supposed 


species of pigeons to breed freely under domestication ; 
these supposed species being quite unknown in a wild 
state, and their becoming nowhere feral ; these species 
having very abnormal characters in certain respects, as 
compared with all other Columbidse, though so like in 
most other respects to the rock-pigeon ; the blue colour 
and various marks occasionally appearing in all the 
breeds, both when kept pure and when crossed; the 
mongrel offspring being perfectly fertile ; — from these 
several reasons, taken together, I can feel no doubt that 
all our domestic breeds have descended from the Co- 
lumba livia with its geographical sub-species. 

In favour of tins view, I may add, firstly, that C. livia, 
or the rock-pigeon, has been found capable of domestica- 
tion in Europe and in India ; and that it agrees in habits 
and in a great number of points of structure with all 
the domestic breeds. Secondly, although an English 
carrier or short-faced tumbler differs immensely in cer- 
tain characters from the rock-pigeon, yet by comparing 
the several sub-breeds of these breeds, more especially 
those brought from distant countries, we can make an 
almost perfect series between the extremes of structure. 
Thirdly, those characters which are mainly distinctive of 
each breed, for instance the wattle and length of beak 
of the carrier, the shortness of that of the tumbler, and 
the number of tail-feathers in the fantail, are in each 
breed eminently variable ; and the explanation of this 
fact will be obvious when we come to treat of selection. 
Fourthly, pigeons have been watched, and tended with 
the utmost care, and loved by many people. They have 
been domesticated for thousands of years in several 
quarters of the world ; the earliest known record of 
pigeons is in the fifth Egyptian dynasty, about 3000 B.C., 
as was pointed out to me by Professor Lepsius; but 
Mr. Birch informs me that pigeons are given in a bill 



of fare in the previous dynasty. In the time of the 
Komans, as we hear from Pliny, immense prices were 
given for pigeons ; " nay, they are come to this pass, that 
they can reckon up their pedigree and race." Pigeons 
were much valued by Akber Khan in India, about the 
year 1600 ; never less than 20,000 pigeons were taken 
with the court. " The monarchs of Iran and Turan sent 
him some very rare birds ;" and, continues the courtly 
historian, "His Majesty by crossing the breeds, which 
method was never practised before, has improved them 
astonishingly." About this same period the Dutch were 
as eager about pigeons as were the old Eomans. The 
paramount importance of these considerations in ex- 
plaining the immense amount of variation which pigeons 
have undergone, will be obvious when we treat of Selec- 
tion. We shall then, also, see how it is that the breeds 
so often have a somewhat monstrous character. It is 
also a most favourable circumstance for the production 
of distinct breeds, that male and female pigeons can be 
easily mated for life ; and thus different breeds can be 
kept together in the same aviary. 

I have discussed the probable origin of domestic 
pigeons at some, yet quite insufficient, length ; because 
when I first kept pigeons and watched the several kinds, 
knowing well how true they bred, I felt fully as much 
difficulty in believing that they could ever have descended 
from a common parent, as any naturalist could in coming 
to a similar conclusion in regard to the many species of 
finches, or other large groups of birds, in nature. One 
circumstance has struck me much ; namely, that all 
the breeders of the various domestic animals and the 
cultivators of plants, with whom I have ever conversed, 
or whose treatises I have read, are firmly convinced 
that the several breeds to which each has attended, are 
descended from so many aboriginally distinct species. 


Ask, as I have asked, a celebrated raiser of Hereford 
cattle, whether his cattle might not have descended 
from long-horns, and he will laugh you to scorn. I 
have never met a pigeon, or poultry, or duck, or 
rabbit fancier, who was not fully convinced that each 
main breed was descended from a distinct species. Van 
Mons, in his treatise on pears and apples, shows how 
utterly he disbelieves that the several sorts, for instance 
a Kibston-pippin or Codlin-apple, could ever have pro- 
ceeded from the seeds of the same tree. Innumerable 
other examples could be given. The explanation, I 
think, is simple: from long-continued study they are 
strongly impressed with the differences between the 
several races ; and though they well know that each 
race varies slightly, for they win their prizes by select- 
ing such slight differences, yet they ignore all general 
arguments, and refuse to sum up in their minds slight 
differences accumulated during many successive genera- 
tions. May not those naturalists who, knowing far less 
of the laws of inheritance than does the breeder, and 
knowing no more than he does of the intermediate links 
in the long lines of descent, yet admit that many of our 
domestic races have descended from the same parents — 
may they not learn a lesson of caution, when they de- 
ride the idea of species in a state of nature being lineal 
descendants of other species? 

Selection. — Let us now briefly consider the steps by 
which domestic races have been produced, either from 
one or from several allied species. Some little effect may, 
perhaps, be attributed to the direct action of the external 
conditions of life, and some little to habit ; but he would 
be a bold man who would account by such agencies for 
the differences of a dray and race horse, a greyhound 
and bloodhound, a carrier and tumbler pigeon. One of 
the most remarkable features in our domesticated races 


is that we see in them adaptation, not indeed to the 
animal's or plant's own good, bnt to man's use or fancy. 
Some variations useful to him have probably arisen 
suddenly, or by one step ; many botanists, for instance, 
believe that the fuller's teazle, with its hooks, which 
cannot be rivalled by any mechanical contrivance, is 
only a variety of the wild Dipsacus ; and this amount of 
change may have suddenly arisen in a seedling. So it 
has probably been with the turnspit dog ; and this is 
known to have been the case with the ancon sheep. But 
when we compare the dray-horse and race-horse, the 
dromedary and camel, the various breeds of sheep fitted 
either for cultivated land or mountain pasture, with the 
wool of one breed good for one purpose, and that of 
another breed for another purpose ; when we compare 
the many breeds of dogs, each good for man in very 
different ways ; when we compare the game-cock, so 
pertinacious in battle, with other breeds so little quarrel- 
some, with " everlasting layers " which never desire to 
sit, and with the bantam so small and elegant; when 
we compare the host of agricultural, culinary, orchard, 
and flower-garden races of plants, most useful to man at 
different seasons and for different purposes, or so beau- 
tiful in his eyes, we must, I think, look further than to 
mere variability. We cannot suppose that all the breeds 
were suddenly produced as perfect and as useful as we 
now see them ; indeed, in several cases, we know that 
this has not been their history. The key is man's power 
of accumulative selection : nature gives successive varia- 
tions ; man adds them up in certain directions useful to 
him. In this sense he may be said to make for himself 
useful breeds. 

The great power of this principle of selection is not 
hypothetical. It is certain that several of our eminent 
breeders have, even within a single lifetime, modified to 


a large extent some breeds of cattle and sheep. In 
order fully to realise what they have done, it is almost 
necessary to read several of the many treatises devoted 
to this subject, and to inspect the animals. Breeders 
habitually speak of an animal's organisation as some- 
thing quite plastic, which they can model almost as they 
please. If I had space I could quote numerous passages 
to this effect from highly competent authorities. Youatt, 
who was probably better acquainted with the works of 
agriculturists than almost any other individual, and who 
was himself a very good judge of an animal, speaks of 
the principle of selection as " that which enables the 
agriculturist, not only to modify the character of his 
flock, but to change it altogether. It is the magician's 
wand, by means of which he may summon into life what- 
ever form and mould he pleases." Lord Somerville, 
speaking of what breeders have done for sheep, says: — 
" It would seem as if they had chalked out upon a wall 
a form perfect in itself, and then had given it existence." 
That most skilful breeder, Sir John Sebright, used to 
say, with respect to pigeons, that "he would produce 
any given feather in three years, but it would take him 
six years to obtain head and beak." In Saxony the im- 
portance of the principle of selection in regard to merino 
sheep is so fully recognised, that men follow it as a 
trade : the sheep are placed on a table and are studied, 
like a picture by a connoisseur ; this is done three times 
at intervals of months, and the sheep are each time 
marked and classed, so that the very best may ultimately 
be selected for breeding. 

What English breeders have actually effected is 
proved by the enormous prices given for animals with a 
good pedigree ; and these have now been exported to 
almost every quarter of the world. The improvement is 
by no means generally due to crossing different breeds ; 


all the best breeders are strongly opposed to this prac- 
tice, except sometimes amongst closely allied sub-breeds. 
And when a cross has been made, the closest selection is 
far more indispensable even than in ordinary cases. If 
selection consisted merely in separating some very dis- 
tinct variety, and breeding from it, the principle would 
be so obvious as hardly to be worth notice ; but its im- 
portance consists in the great effect produced by the 
accumulation in one direction, during successive gene- 
rations, of differences absolutely inappreciable by an 
uneducated eye — differences which I for one have vainly 
attempted to appreciate. Not one man in a thousand 
has accuracy of eye and judgment sufficient to become 
an eminent breeder. If gifted with these qualities, and 
he studies his subject for years, and devotes his lifetime 
to it with indomitable perseverance, he will succeed, and 
may make great improvements ; if he wants any of these 
qualities, he will assuredly fail. Few would readily 
believe in the natural capacity and years of practice 
requisite to become even a skilful pigeon-fancier. 

The same principles are followed by horticulturists ; 
but the variations are here often more abrupt. No one 
supposes that our choicest productions have been pro- 
duced by a single variation from the aboriginal stock. 
We have proofs that this is not so in some cases, in which 
exact records have been kept ; thus, to give a very 
trifling instance, the steadily-increasing size of the com- 
mon gooseberry may be quoted. We see an astonishing 
improvement in many florists' flowers, when the flowers of 
the present day are compared with drawings made only 
twenty or thirty years ago. When a race of plants is 
once pretty well established, the seed-raisers do not pick 
out the best plants, but merely go over their seed-beds, 
and pull up the " rogues," as they call the plants that 
deviate from the proper standard. With animals this 


kind of selection is, in fact, also followed ; for hardly any 
one is so careless as to allow his worst animals to breed. 

In regard to plants, there is another means of observ- 
ing the accumulated effects of selection — namely, by 
comparing the diversity of flowers in the different varie- 
ties of the same species in the flower-garden ; the diversity 
of leaves, pods, or tubers, or whatever part is valued, in 
the kitchen-garden, in comparison with the flowers of the 
same varieties ; and the diversity of fruit of the same 
species in the orchard, in comparison with the leaves and 
flowers of the same set of varieties. See how different 
the leaves of the cabbage are, and how extremely alike 
the flowers ; how unlike the flowers of the heartsease are, 
and how alike the leaves ; how much the fruit of the 
different kinds of gooseberries differ in size, colour, shape, 
and hairiness, and yet the flowers present very slight 
differences. It is not that the varieties which differ 
largely in some one point do not differ at all in other 
points ; this is hardly ever, perhaps never, the case. 
The laws of correlation of growth, the importance of 
which should never be overlooked, will ensure some dif- 
ferences; but, as a general rule, I cannot doubt that 
the continued selection of slight variations, either in the 
leaves, the flowers, or the fruit, will produce races 
differing from each other chiefly in these characters. 

It may be objected that the principle of selection has 
been reduced to methodical practice for scarcely more 
than three-quarters of a century ; it has certainly been 
more attended to of late years, and many treatises have 
been published on the subject ; and the result, I may 
add, has been, in a corresponding degree, rapid and 
important. But it is very far from true that the prin- 
ciple is a modern discovery. I could give several refer- 
ences to the full acknowledgment of the importance of 
the principle in works of high antiquity. In rude and 



barbarous periods of English history choice animals were 
often imported, and laws were passed to prevent their 
exportation : the destruction of horses under a certain 
size was ordered, and this may be compared to the 
" roguing " of plants by nurserymen. The principle of 
selection I find distinctly given in an ancient Chinese 
encyclopaedia. Explicit rules are laid down by some of 
the Koman classical writers. From passages in Genesis, 
it is clear that the colour of domestic animals was at that 
early period attended to. Savages now sometimes cross 
their dogs with wild canine animals, to improve the 
breed, and they formerly did so, as is attested by passages 
in Pliny. The savages in South Africa match their 
draught cattle by colour, as do some of the Esquimaux 
their teams of dogs. Livingstone shows how much good 
domestic breeds are valued by the negroes of the in- 
terior of Africa who have not associated with Europeans. 
Some of these facts do not show actual selection, but 
they show that the breeding of domestic animals was 
carefully attended to in ancient times, and is now 
attended to by the lowest savages. It would, indeed, 
have been a strange fact, had attention not been paid to 
breeding, for the inheritance of good and bad qualities is 
so obvious. 

At the present time, eminent breeders try by me- 
thodical selection, with a distinct object in view, to make 
a new strain or sub-breed, superior to anything existing 
in the country. But, for our purpose, a kind of Selec- 
tion, which may be called Unconscious, and which results 
from every one trying to possess and breed from the best 
individual animals, is more important. Thus, a man 
who intends keeping pointers naturally tries to get as 
good dogs as he can, and afterwards breeds from his own 
best dogs, but he has no wish or expectation of per- 
manently altering the breed. Nevertheless I cannot 


doubt that this process, continued during centuries, 
would improve and modify any breed, in the same way 
as Bakewell, Collins, &c, by this very same process, 
only carried on more methodically, did greatly modify, 
even during their own lifetimes, the forms and qualities 
of their cattle. Slow and insensible changes of this 
kind could never be recognised unless actual measure- 
ments or careful drawings of the breeds in question 
had been made long ago, which might serve for com- 
parison. In some cases, however, unchanged or but 
little changed individuals of the same breed may be found 
in less civilised districts, where the breed has been less 
improved. There is reason to believe that King Charles's 
spaniel has been unconsciously modified to a large extent 
since the time of that monarch. Some highly competent 
authorities are convinced that the setter is directly derived 
from the spaniel, and has probably been slowly altered 
from it. It is known that the English pointer has been 
greatly changed within the last century, and in this 
case the change has, it is believed, been chiefly effected 
by crosses with the fox-hound; but what concerns us 
is, that the change has been effected unconsciously and 
gradually, and yet so effectually, that, though the old 
Spanish pointer certainly came from Spain, Mr. Borrow 
has not seen, as I am informed by him, any native dog 
in Spain like our pointer. 

By a similar process of selection, and by careful train- 
ing, the whole body of English racehorses have come to 
surpass in fleetness and size the parent Arab stock, so 
that the latter, by the regulations for the Goodwood Kaces, 
are favoured in the weights they carry. Lord Spencer 
and others have shown how the cattle of England have 
increased in weight and in early maturity, compared with 
the stock formerly kept in this country. By comparing 
the accounts given in old pigeon treatises of carriers 


and tumblers with these breeds as now existing in 
Britain, India, and Persia, we can, I think, clearly trace 
the stages through which they have insensibly passed, 
and come to differ so greatly from the rock-pigeon. 

Youatt gives an excellent illustration of the effects of 
a course of selection, which may be considered as un- 
consciously followed, in so far that the breeders could 
never have expected or even have wished to have pro- 
duced the result which ensued — namely, the production 
of two distinct strains. The two flocks of Leicester sheep 
kept by Mr. Buckley and Mr. Burgess, as Mr. Youatt re- 
marks, " have been purely bred from the original stock of 
Mr. Bakewell for upwards of fifty years. There is not a 
suspicion existing in the mind of any one at all ac- 
quainted with the subject that the owner of either of 
them has deviated in any one instance from the pure 
blood of Mr. Bakewell's flock, and yet the difference 
between the sheep possessed by these two gentlemen is 
so great that they have the appearance of being quite 
different varieties." 

If there exist savages so barbarous as never to think 
of the inherited character of the offspring of their 
domestic animals, yet any one animal particularly useful 
to them, for any special purpose, would be carefully 
preserved during famines and other accidents, to which 
savages are so liable, and such choice animals would thus 
generally leave more offspring than the inferior ones; 
so that in this case there would be a kind of uncon- 
scious selection going on. We see the value set on 
animals even by the barbarians of Tierra del Fuego, by 
their killing and devouring their old women, in times of 
dearth, as of less value than their dogs. 

In plants the same gradual process of improvement, 
through the occasional preservation of the best indi- 
viduals, whether or not sufficiently distinct to be ranked 


at their first appearance as distinct varieties, and whether 
or not two or more species or races have become 
blended together by crossing, may plainly be recognised 
in the increased size and beauty which we now see in the 
varieties of the heartsease, rose, pelargonium, dahlia, and 
other plants, when compared with the older varieties or 
with their parent-stocks. No one would ever expect to 
get a first-rate heartsease or dahlia from the seed of a 
wild plant. No one would expect to raise a first-rate 
melting pear from the seed of the wild pear, though 
he might succeed from a poor seedling growing wild, 
if it had come from a garden-stock. The pear, though 
cultivated in classical times, appears, from Pliny's de- 
scription, to have been a fruit of very inferior quality. 
I have seen great surprise expressed in horticultural works 
at the wonderful skill of gardeners, in having produced 
such splendid results from such poor materials ; but the 
art, I cannot doubt, has been simple, and, as far as the 
final result is concerned, has been followed almost un- 
consciously. It has consisted in always cultivating the 
best known variety, sowing its seeds, and, when a slightly 
better variety has chanced to appear, selecting it, and 
so onwards. But the gardeners of the classical period, 
who cultivated the best pear they could procure, never 
thought what solendid fruit we should eat ; though we 
owe our excellent fruit, in some small degree, to their 
having naturally chosen and preserved the best varieties 
they could anywhere find. 

A large amount of change in our cultivated plants, 
thus slowly and unconsciously accumulated, explains, as 
I believe, the well-known fact, that in a vast number of 
cases we cannot recognise, and therefore do not know, 
the wild parent-stocks of the plants which have been 
longest cultivated in our flower and kitchen gardens. 
If it has taken centuries or thousands of years to improve 


or modify most of our plants up to their present standard 
of usefulness to man, we can understand how it is that 
neither Australia, the Cape of Good Hope, nor any other 
region inhabited by quite uncivilised man, has afforded us 
a single plant worth culture. It is not that these coun- 
tries, so rich in species, do not by a strange chance possess 
the aboriginal stocks of any useful plants, but that the 
native plants have not been improved by continued se- 
lection up to a standard of perfection comparable with 
that given to the plants in countries anciently civilised. 

In regard to the domestic animals kept by uncivilised 
man, it should not be overlooked that they almost 
always have to struggle for their own food, at least 
during certain seasons. And in two countries very dif- 
ferently circumstanced, individuals of the same species, 
having slightly different constitutions or structure, would 
often succeed better in the one country than in the 
other, and thus by a process of " natural selection," as 
will hereafter be more fully explained, two sub-breeds 
might be formed. This, perhaps, partly explains what 
has been remarked by some authors, namely, that the 
varieties kept by savages have more of the character of 
species than the varieties kept in civilised countries. 

On the view here given of the all-important part which 
selection by man has played, it becomes at once obvious, 
how it is that our domestic races show adaptation in their 
structure or in their habits to man's wants or fancies. 
We can, I think, further understand the frequently 
abnormal character of our domestic races, and likewise 
their differences being so great in external characters 
and relatively so slight in internal parts or organs. 
Man can hardly select, or only with much difficulty, any 
deviation of structure excepting such as is externally 
visible ; and indeed he rarely cares for what is internal. 
He can never act by selection, excepting on variations 


which are first given to him in some slight degree by 
nature. No man would ever try to make a fantail, till 
he saw a pigeon with a tail developed in some slight de- 
gree in an unusual manner, or a pouter till he saw a 
pigeon with a crop of somewhat unusual size ; and the 
more abnormal or unusual any character was when it first 
appeared, the more likely it would be to catch his atten- 
tion. But to use such an expression as trying to make 
a fantail, is, I have no doubt, in most cases, utterly in- 
correct. The man who first selected a pigeon with a 
slightly larger tail, never dreamed what the descendants 
of that pigeon would become through long-continued, 
partly unconscious and partly methodical selection. Per- 
haps the parent bird of all fantails had only fourteen tail- 
feathers somewhat expanded, like the present Java fantail, 
or like individuals of other and distinct breeds, in which 
as many as seventeen tail-feathers have been counted. 
Perhaps the first pouter-pigeon did not inflate its crop 
much more than the turbit now does the upper part of 
its oesophagus, — a habit which is disregarded by all 
fanciers, as it is not one of the points of the breed. 

Nor let it be thought that some great deviation of 
structure would be necessary to catch the fancier's eye : 
he perceives extremely small differences, and it is in 
human nature to value any novelty, however slight, in 
one's own possession. Nor must the value which would 
formerly be set on any slight differences in the individuals 
of the same species, be judged of by the value which 
would now be set on them, after several breeds have 
once fairly been established. Many slight differences 
might, and indeed do now, arise amongst pigeons, which 
are rejected as faults or deviations from the standard of 
perfection of each breed. The common goose has not 
given rise to any marked varieties ; hence the Thoulouse 
and the common breed, which differ only in colour, that 


most fleeting of characters, have lately been exhibited 
as distinct at our poultry-shows. 

I think these views further explain what has sometimes 
been noticed — namely, that we know nothing about the 
origin or history of any of our domestic breeds. But, in 
fact, a breed, like a dialect of a language, can hardly be 
said to have had a definite origin. A man preserves and 
breeds from an individual with some slight deviation of 
structure, or takes more care than usual in matching his 
best animals and thus improves them, and the improved 
individuals slowly spread in the immediate neighbour- 
hood. But as yet they will hardly have a distinct name, 
and from being only slightly valued, their history will 
be disregarded. When further improved by the same 
slow and gradual process, they will spread more widely, 
and will get recognised as something distinct and valu- 
able, and will then probably first receive a provincial 
name. In semi-civilised countries, with little free com- 
munication, the spreading and knowledge of any new 
sub-breed will be a slow process. As soon as the points of 
value of the new sub-breed are once fully acknowledged, 
the principle, as I have called it, of unconscious selection 
will always tend, — perhaps more at one period than at 
another, as the breed rises or falls in fashion, — perhaps 
more in one district than in another, according to the 
state of civilisation of the inhabitants, — slowly to add to 
the characteristic features of the breed, whatever they 
may be. But the chance will be infinitely small of any 
record having been preserved of such slow, varying, and 
insensible changes. 

I must now say a few words on the circumstances, 
favourable, or the reverse, to man's power of selection. 
A high degree of variability is obviously favourable, 
as freely giving the materials for selection to work 
on ; not that mere individual differences are not amply 


sufficient, with extreme care, to allow of the accumulation 
of a large amount of modification in almost any desired 
direction. But as variations manifestly useful or pleasing 
to man appear only occasionally, the chance of their ap- 
pearance will be much increased by a large number of 
individuals being kept ; and hence this comes to be of 
the highest importance to success. On this principle 
Marshall has remarked, with respect to the sheep of parts 
of Yorkshire, that " as they generally belong to poor 
people, and are mostly in small lots, they never can be 
improved." On the other hand, nurserymen, from raising 
large stocks of the same plants, are generally far more 
successful than amateurs in getting new and valuable 
varieties. The keeping of a large number of individuals 
of a species in any country requires that the species 
should be placed under favourable conditions of life, so 
as to breed freely in that country. When the individuals 
of any species are scanty, all the individuals, whatever 
their quality may be, will generally be allowed to breed, 
and this will effectually prevent selection. But probably 
the most important point of all, is, that the animal or 
plant should be so highly useful to man, or so much 
valued by him, that the closest attention should be 
paid to even the slightest deviation in the qualities or 
structure of each individual. Unless such attention 
be paid nothing can be effected. I have seen it gravely 
remarked, that it was most fortunate that the straw- 
berry began to vary just when gardeners began to attend 
closely to this plant. No doubt the strawberry had 
always varied since it was cultivated, but the slight 
varieties had been neglected. As soon, however, as 
gardeners picked out individual plants with slightly 
larger, earlier, or better fruit, and raised seedlings from 
them, and again picked out the best seedlings and 
bred from them, then, there appeared (aided by some 


crossing with distinct species) those many admirable 
varieties of the strawberry which have been raised 
during the last thirty or forty years. 

In the case of animals with separate sexes, facility 
in preventing crosses is an important element of success 
in the formation of new races, — at least, in a country 
which is already stocked with other races. In this re- 
spect enclosure of the land plays a part. Wandering 
savages or the inhabitants of open plains rarely possess 
more than one breed of the same species. Pigeons can 
be mated for life, and this is a great convenience to the 
fancier, for thus many races may be kept true, though 
mingled in the same aviary ; and this circumstance must 
have largely favoured the improvement and formation 
of new breeds. Pigeons, I may add, can be propagated 
in great numbers and at a very quick rate, and inferior 
birds may be freely rejected, as when killed they serve 
for food. On the other hand, cats, from their nocturnal 
rambling habits, cannot be matched, and, although so 
much valued by women and children, we hardly ever 
see a distinct breed kept up ; such breeds as we do 
sometimes see are almost always imported from some 
other country, often from islands. Although I do not 
doubt that some domestic animals vary less than others, 
yet the rarity or absence of distinct breeds of the cat, 
the donkey, peacock, goose, &c, may be attributed in 
main part to selection not having been brought into 
play : in cats, from the difficulty in pairing them ; in 
donkeys, from only a few being kept by poor j^eople, and 
little attention paid to their breeding ; in peacocks, from 
not being very easily reared and a large stock not kept ; 
in geese, from being valuable only for two purposes, food 
and feathers, and more especially from no pleasure hav- 
ing been felt in the display of distinct breeds. 


To sum up on the origin of our Domestic Kaces of 
animals and plants. I believe that the conditions of 
life, from their action on the reproductive system, are 
so far of the highest importance as causing variability. 
I do not believe that variability is an inherent and 
necessary contingency, under all circumstances, with all 
organic beings, as some authors have thought. The 
effects of variability are modified by various degrees of 
inheritance and of reversion. Variability is governed 
by many unknown laws, more especially by that of cor- 
relation of growth. Something may be attributed to 
the direct action of the conditions of life. Something 
must be attributed to use and disuse. The final result 
is thus rendered infinitely complex. In some cases, I 
do not doubt that the intercrossing of species, aborigin- 
ally distinct, has played an important part in the origin 
of our domestic productions. When in any country 
several domestic breeds have once been established, 
their occasional intercrossing, with the aid of selection, 
has, no doubt, largely aided in the formation of new 
sub-breeds; but the importance of the crossing of 
varieties has, I believe, been greatly exaggerated, both 
in regard to animals and to those plants which are pro- 
pagated by seed. In plants which are temporarily pro- 
pagated by cuttings, buds, &c, the importance of the 
crossing both of distinct species and of varieties is im- 
mense ; for the cultivator here quite disregards the ex- 
treme variability both of hybrids and mongrels, and the 
frequent sterility of hybrids ; but the cases of plants not 
propagated by seed are of little importance to us, for 
their endurance is only temporary. Over all these 
causes of Change I am convinced that the accumu- 
lative action of Selection, whether applied methodically 
and more quickly, or unconsciously and more slowly, 
but more efficiently, is by far the predominant Power. 



Variation under Nature. 

Variability — Individual differences — Doubtful species — Wide 
ranging, much diffused, and common species vary most — Spe- 
cies of the larger genera in any country vary more than the species 
of the smaller genera— Many of the species of the larger genera 
resemble varieties in being very closely, but unequally, related 
to each other, and in having restricted ranges. 

Before applying the principles arrived at in the last 
chapter to organic beings in a state of nature, we must 
briefly discuss whether these latter are subject to any 
variation. To treat this subject at all properly, a long 
catalogue of dry facts should be given ; but these I shall 
reserve for my future work. Nor shall I here discuss 
the various definitions which have been given of the 
term species. No one definition has as yet satisfied all 
naturalists ; yet every naturalist knows vaguely what 
he means when he speaks of a species. Generally the 
term includes the unknown element of a distinct act of 
creation. The term " variety " is almost equally difficult 
to define; but here community of descent is almost 
universally implied, though it can rarely be proved. 
We have also what are called monstrosities ; but they 
graduate into varieties. By a monstrosity I presume is 
meant some considerable deviation of structure in one 
part, either injurious to or not useful to the species, and 
not generally propagated. Some authors use the term 
"variation" in a technical sense, as implying a modifica- 
tion directly due to the physical conditions of life ; and 
" variations " in this sense are supposed not to be in- 
herited : but who can say that the dwarfed condition of 
shells in the brackish waters of the Baltic, or dwarfed 


plants on Alpine summits, or the thicker fur of an 
animal from far northwards, would not in some cases be 
inherited for at least some few generations ? and in this 
case I presume that the form would be called a variety. 
Again, we have many slight differences which may be 
called individual differences, such as are known fre- 
quently to appear in the offspring from the same parents, 
or which may be presumed to have thus arisen, from 
being frequently observed in the individuals of the same 
species inhabiting the same confined locality. No one 
supposes that all the individuals of the same species are 
cast in the very same mould. These individual differ- 
ences are highly important for us, as they afford mate- 
rials for natural selection to accumulate, in the same 
manner as man can accumulate in any given direction 
individual differences in his domesticated productions. 
These individual differences generally affect what natu- 
ralists consider unimportant parts ; but I could show by 
a long catalogue of facts, that parts which must be called 
important, whether viewed under a physiological or clas- 
sificatory point of view, sometimes vary in the indivi- 
duals of the same species. I am convinced that the most 
experienced naturalist would be surprised at the number 
of the cases of variability, even in important parts of 
structure, which he could collect on good authority, as I 
have collected, during a course of years. It should be 
remembered that systematists are far from pleased at 
finding variability in important characters, and that there 
are not many men who will laboriously examine internal 
and important organs, and compare them in many speci- 
mens of the same species. I should never have expected 
that the branching of the main nerves close to the great 
central ganglion of an insect would have been variable 
in the same species; I should have expected that 
changes of this nature could have been effected only 


by slow degrees : yet quite recently Mr. Lubbock has 
shown a degree of variability in these main nerves in 
Coccus, which may almost be compared to the irregular 
branching of the stem of a tree. This philosophical 
naturalist, I may add, has also quite recently shown 
that the muscles in the larvae of certain insects are 
very far from uniform. Authors sometimes argue in 
a circle when they state that important organs never 
vary ; for these same authors practically rank that cha- 
racter as important (as some few naturalists have honestly 
confessed) which does not vary ; and, under this point 
of view, no instance of an important part varying will 
ever be found : but under any other point of view many 
instances assuredly can be given. 

There is one point connected with individual differ- 
ences, which seems to me extremely perplexing: I 
refer to those genera which have sometimes been called 
" protean" or " polymorphic," in which the species present 
an inordinate amount of variation I and hardly two natu- 
ralists can agree which forms to rank as species and 
which as varieties. We may instance Kubus, Kosa, and 
Hieracium amongst plants, several genera of insects, and 
several genera of Brachiopod shells. In most polymorphic 
genera some of the species have fixed and definite cha- 
racters. Genera which are polymorphic in one country 
seem to be, with some few exceptions, polymorphic in 
other countries, and likewise, judging from Brachiopod 
shells, at former periods of time. These facts seem to 
be very perplexing, for they seem to show that this kind 
of variability is independent of the conditions of life. 
I am inclined to suspect that we see in these poly- 
morphic genera variations in points of structure which are 
of no service or disservice to the species, and which con- 
sequently have not been seized on and rendered definite 
by natural selection, as hereafter will be explained. 


Those forms which possess in some considerable 
degree the character of species, but which are so closely- 
similar to some other forms, or are so closely linked to 
them by intermediate gradations, that naturalists do not 
like to rank them as distinct species, are in several re- 
spects the most important for us. We have every reason 
to believe that many of these doubtful and closely-allied 
forms have permanently retained their characters in 
their own country for a long time ; for as long, as far 
as we know, as have good and true species. Practi- 
cally, when a naturalist can unite two forms together 
by others having intermediate characters, he treats the 
one as a variety of the other, ranking the most common, 
but sometimes the one first described, as the species, 
and the other as the variety. But cases of great diffi- 
culty, which I will not here enumerate, sometimes 
occur in deciding whether or not to rank one form as 
a variety of another, even when they are closely con- 
nected by intermediate links ; nor will the commonly- 
assumed hybrid nature of the intermediate links always 
remove the difficulty. In very many cases, however, 
one form is ranked as a variety of another, not because 
the intermediate links have actually been found, but 
because analogy leads the observer to suppose either 
that they do now somewhere exist, or may formerly 
have existed ; and here a wide door for the entry of 
doubt and conjecture is opened. 

Hence, in determining whether a form should be 
ranked as a species or a variety, the opinion of natural- 
ists having sound judgment and wide experience seems 
the only guide to follow. We must, however, in many 
cases, decide by a majority of naturalists, for few well- 
marked and well-known varieties can be named which 
have not been ranked as species by at least some com- 
petent judges. 


That varieties of this doubtful nature are far from 
uncommon cannot be disputed. Compare the several 
floras of Great Britain, of France or of the United 
States, drawn up by different botanists, and see what a 
surprising number of forms have been ranked by one 
botanist as good species, and by another as mere 
varieties. Mr. H. C. Watson, to whom I lie under 
deep obligation for assistance of all kinds, has marked 
for me 182 British plants, which are generally con- 
sidered as varieties, but which have all been ranked 
by botanists as species ; and in making this list he 
has omitted many trifling varieties, but which never- 
theless have been ranked by some botanists as species, 
and he has entirely omitted several highly polymorphic 
genera. Under genera, including the most polymorphic 
forms, Mr. Babington gives 251 species, whereas Mr. 
Bentham gives only 112, — a difference of 139 doubtful 
forms ! Amongst animals which unite for each birth, 
and which are highly locomotive, doubtful forms, ranked 
by one zoologist as a species and by another as a variety, 
can rarely be found within the same country, but are 
common in separated areas. How many of those birds 
and insects in North America and Europe, which differ 
very slightly from each other, have been ranked by 
one eminent naturalist as undoubted species, and by 
another as varieties, or, as they are often called, as 
geographical races ! Many years ago, when comparing, 
and seeing others compare, the birds from the sepa- 
rate islands of the Galapagos Archipelago, both one 
with another, and with those from the American main- 
land, I was much struck how entirely vague and arbi- 
trary is the distinction between species and varieties. 
On the islets of the little Madeira group there are 
many insects which are characterized as varieties in 
Mr. Wollaston's admirable work, but which it cannot 


be doubted would be ranked as distinct species by many 
entomologists. Even Ireland has a few animals, now 
generally regarded as varieties, but which have been 
ranked as species by some zoologists. Several most 
experienced ornithologists consider our British red 
grouse as only a strongly-marked race of a Norwegian 
species, whereas the greater number rank it as an 
undoubted species peculiar to Great Britain. A wide 
distance between the homes of two doubtful forms leads 
many naturalists to rank both as distinct species ; but 
what distance, it has been well asked, will suffice? if 
that between America and Europe is ample, will that 
between the Continent and the Azores, or Madeira, or 
the Canaries, or Ireland, be sufficient? It must be 
admitted that many forms, considered by highly-compe- 
tent judges as varieties, have so perfectly the character 
of species that they are ranked by other highly compe- 
tent judges as good and true species. But to discuss 
whether they are rightly called species or varieties, 
before any definition of these terms has been generally 
accepted, is vainly to beat the air. 

Many of the cases of strongly-marked varieties or 
doubtful species well deserve consideration ; for several 
interesting lines of argument, from geographical dis- 
tribution, analogical variation, hybridism, &c, have 
been brought to bear on the attempt to determine their 
rank. I will here give only a single instance, — the 
well-known one of the primrose and cowslip, or Primula 
veris and elatior. These plants differ considerably in 
appearance ; they have a different flavour and emit a 
different odour ; they flower at slightly different periods ; 
they grow in somewhat different stations ; they ascend 
mountains to different heights ; they have different 
geographical ranges ; and lastly, according to very 
numerous experiments made during several years by 



that most careful observer Gartner, they can be crossed 
only with much difficulty. We could hardly wish for 
better evidence of the two forms being specifically dis- 
tinct. On the other hand, they are united by many 
intermediate links, and it is very doubtful whether these 
links are hybrids ; and there is, as it seems to me, an 
overwhelming amount of experimental evidence, show- 
ing that they descend from common parents, and con- 
sequently must be ranked as varieties. 

Close investigation, in most cases, will bring naturalists 
to an agreement how to rank doubtful forms. Yet it 
must be confessed, that it is in the best-known countries 
that we find the greatest number of forms of doubtful 
value. I have been struck with the fact, that if any 
animal or plant in a state of nature be highly useful 
to man, or from any cause closely attract his attention, 
varieties of it will almost universally be found recorded. 
These varieties, moreover, will be often ranked by some 
authors as species. Look at the common oak, how 
closely it has been studied ; yet a German author 
makes more than a dozen species out of forms, which 
are very generally considered as varieties ; and in this 
country the highest botanical authorities and practical 
men can be quoted to show that the sessile and pedun- 
culated oaks are either good and distinct species or 
mere varieties. 

When a young naturalist commences the study of a 
group of organisms quite unknown to him, he is at first 
much perplexed to determine what differences to consider 
as specific, and what as varieties ; for he knows nothing 
of the amount and kind of variation to which the group 
is subject ; and this shows, at least, how very generally 
there is some variation. But if he confine his attention 
to one class within one country, he will soon make up 
his mind how to rank most of the doubtful forms. His 


general tendency will be to make many species, for he 
will become impressed, just like the pigeon or poultry- 
fancier before alluded to, with the amount of difference 
in the forms which he is continually studying ; and he 
has little general knowledge of analogical variation in 
other groups and in other countries, by which to correct 
Ins first impressions. As he extends the range of his 
observations, he will meet with more cases of difficulty ; 
for he will encounter a greater number of closely-allied 
forms. But if his observations be widely extended, he 
will in the end generally be enabled to make up his own 
mind which to call varieties and which species ; but he 
will succeed in this at the expense of admitting much 
variation, — and the truth of this admission will often be 
disputed by other naturalists. When, moreover, he 
comes to study allied forms brought from countries not 
now continuous, in which case he can hardly hope to 
find the intermediate links between his doubtful forms, 
he will have to trust almost entirely to analogy, and his 
difficulties will rise to a climax. 

Certainly no clear line of demarcation has as yet 
been drawn between species and sub-species — that is, 
the forms which in the opinion of some naturalists come 
very near to, but do not quite arrive at the rank of 
species ; or, again, between sub-species and well-marked 
varieties, or between lesser varieties and individual dif- 
ferences. These differences blend into each other in an 
insensible series ; and a series impresses the mind with 
the idea of an actual passage. 

Hence I look at individual differences, though of 
small interest to the systematist, as of high importance 
for us, as being the first step towards such slight varieties 
as are barely thought worth recording in works on natural 
history. And I look at varieties which are in any degree 
more distinct and permanent, as steps leading to more 



strongly marked and more permanent varieties ; and at 
these latter, as leading to sub-species, and to species. 
The passage from one stage of difference to another and 
higher stage may be, in some cases, due merely to the 
long-continued action of different physical conditions in 
two different regions; but I have not much faith in 
this view ; and I attribute the passage of a variety, from 
a state in which it differs very slightly from its parent 
to one in which it differs more, to the action of natural 
selection in accumulating (as will hereafter be more fully 
explained) differences of structure in certain definite 
directions. Hence I believe a well-marked variety may be 
justly called an incipient species ; but whether this belief 
be justifiable must be judged of by the general weight of 
the several facts and views given throughout this work. 

It need not be supposed that all varieties or incipient 
species necessarily attain the rank of species. They may 
whilst in this incipient state become extinct, or they may 
endure as varieties for very long periods, as has been 
shown to be the case by Mr. Wollaston with the varie- 
ties of certain fossil land-shells in Madeira. If a variety 
were to flourish so as to exceed in numbers the parent 
species, it would then rank as the species, and the spe- 
cies as the variety ; or it might come to supplant and 
exterminate the parent species ; or both might co-exist, 
and both rank as independent species. But we shall 
hereafter have to return to this subject. 

From these remarks it will be seen that I look at the 
term species, as one arbitrarily given for the sake of con- 
venience to a set of individuals closely resembling each 
other, and that it does not essentially differ from the 
term variety, which is given to less distinct and more 
fluctuating forms. The term variety, again, in com- 
parison with mere individual differences, is also applied 
arbitrarily, and for mere convenience sake. 


Guided by theoretical considerations, I thought that 
some interesting results might be obtained in regard to 
the nature and relations of the species which vary most, 
by tabulating all the varieties in several well-worked 
floras. At first this seemed a simple task ; but Mr. H. 
C. Watson, to whom I am much indebted for valuable 
advice and assistance on this subject, soon convinced 
me that there were many difficulties, as did subse- 
quently Dr. Hooker, even in stronger terms. I shall 
reserve for my future work the discussion of these dif- 
ficulties, and the tables themselves of the proportional 
numbers of the varying species. Dr. Hooker permits 
me to add, that after having carefully read my manu- 
script, and examined the tables, he thinks that the fol- 
lowing statements are fairly well established. The whole 
subject, however, treated as it necessarily here is with 
much brevity, is rather perplexing, and allusions cannot 
be avoided to the " struggle for existence," " divergence 
of character," and other questions, hereafter to be dis- 

Alph. De Candolle and others have shown that plants 
which have very wide ranges generally present varieties ; 
and this might have been expected, as they become ex- 
posed to diverse physical conditions, and as they come 
into competition (which, as we shall hereafter see, is a 
far more important circumstance) with different sets of 
organic beings. But my tables further show that, in 
any limited country, the species which are most common, 
that is abound most in individuals, and the species which 
are most widely diffused within their own country (and 
this is a different consideration from wide range, and to 
a certain extent from commonness), often give rise to 
varieties sufficiently well-marked to have been recorded 
in botanical works. Hence it is the most flourishing, 
or, as they may be called, the dominant species, — 


those which range widely over the world, are the most 
diffused in their own country, and are the most numerous 
in individuals, — which oftenest produce well-marked 
varieties, or, as I consider them, incipient species. And 
this, perhaps, might have been anticipated ; for, as 
varieties, in order to become in any degree permanent, 
necessarily have to struggle with the other inhabitants 
of the country, the species which are already dominant 
will be the most likely to yield offspring which, though 
in some slight degree modified, will still inherit those 
advantages that enabled their parents to become domi- 
nant over their compatriots. 

If the plants inhabiting a country and described in 
any Flora be divided into two equal masses, all those in 
the larger genera being placed on one side, and all those 
in the smaller genera on the other side, a somewhat 
larger number of the very common and much diffused or 
dominant species will be found on the side of the larger 
genera. This, again, might have been anticipated ; for 
the mere fact of many species of the same genus in- 
habiting any country, shows that there is something in 
the organic or inorganic conditions of that country 
favourable to the genus ; and, consequently, we might 
have expected to have found in the larger genera, or 
those including many species, a large proportional num- 
ber of dominant species. But so many causes tend to 
obscure this result, that I am surprised that my tables 
show even a small majority on the side of the larger 
genera. I will here allude to only two causes of 
obscurity. Fresh-water and salt-loving plants have 
generally very wide ranges and are much diffused, 
but this seems to be connected with the nature of the 
stations inhabited by them, and has little or no relation 
to the size of the genera to which the species belong. 
Again, plants low in the scale of organisation are 


generally much more widely diffused than plants higher 
in the scale ; and here again there is no close relation 
to the size of the genera. The cause of lowly-organised 
plants ranging widely will be discussed in our chapter 
on geographical distribution. 

From looking at species as only strongly-marked and 
well-defined varieties, I was led to anticipate that the 
species of the larger genera in each country would oftener 
present varieties, than the species of the smaller genera ; 
for wherever many closely related species (i. e. species of 
the same genus) have been formed, many varieties or 
incipient species ought, as a general rule, to be now 
forming. Where many large trees grow, we expect to 
find saplings. Where many species of a genus have 
been formed through variation, circumstances have been 
favourable for variation ; and hence we might expect 
that the circumstances would generally be still favour- 
able to variation. On the other hand, if we look at 
each species as a special act of creation, there is no 
apparent reason why more varieties should occur in a 
group having many species, than in one having few. 

To test the truth of this anticipation I have arranged 
the plants of twelve countries, and the coleopterous 
insects of two districts, into two nearly equal masses, the 
species of the larger genera on one side, and those of the 
smaller genera on the other side, and it has invariably 
proved to be the case that a larger proportion of the 
species on the side of the larger genera present varieties, 
than on the side of the smaller genera. Moreover, the 
species of the large genera which present any varieties, 
invariably present a larger average number of varieties 
than do the species of the small genera. Both these 
results follow when another division is made, and when 
all the smallest genera, with from only one to four spe- 
cies, are absolutely excluded from the tables. These 


facts are of plain signification on the view that species 
are only strongly marked and permanent varieties ; for 
wherever many species of the same genus have been 
formed, or where, if we may use the expression, the 
manufactory of species has been active, we ought gene- 
rally to find the manufactory still in action, more espe- 
cially as we have every reason to believe the process of 
manufacturing new species to be a slow one. And this 
certainly is the case, if varieties be looked at as inci- 
pient species ; for my tables clearly show as a general 
rule that, wherever many species of a genus have been 
formed, the species of that genus present a number of 
varieties, that is of incipient species, beyond the average. 
It is not that all large genera are now varying much, and 
are thus increasing in the number of their species, or that 
no small genera are now varying and increasing ; for if 
this had been so, it would have been fatal to my theory ; 
inasmuch as geology plainly tells us that small genera 
have in the lapse of time often increased greatly in 
size; and that large genera have often come to their 
maxima, declined, and disappeared. All that we want 
to show is, that where many species of a genus have 
been formed, on an average many are still forming; 
and this holds good. 

There are other relations between the species of large 
genera and their recorded varieties which deserve notice. 
We have seen that there is no infallible criterion by 
which to distinguish species and well-marked varieties ; 
and in those cases in which intermediate links have not 
been found between doubtful forms, naturalists are com- 
pelled to come to a determination by the amount of 
difference between them, judging by analogy whether 
or not the amount suffices to raise one or both to the 
rank of species. Hence the amount of difference is one 
very important criterion in settling whether two forms 


should be ranked as species or varieties. Now Fries 
has remarked in regard to plants, and Westwood in 
regard to insects, that in large genera the amount of 
difference between the species is often exceedingly small. 
I have endeavoured to test this numerically by averages, 
and, as far as my imperfect results go, they always con- 
firm the view. I have also consulted some sagacious 
and most experienced observers, and, after deliberation, 
they concur in this view. In this respect, therefore, the 
species of the larger genera resemble varieties, more 
than do the species of the smaller genera. Or the case 
may be put in another way, and it may be said, that 
in the larger genera, in which a number of varieties or 
incipient species greater than the average are now 
manufacturing, many of the species already manufac- 
tured still to a certain extent resemble varieties, for 
they differ from each other by a less than usual amount 
of difference. 

Moreover, the species of the large genera are related 
to each other, in the same manner as the varieties of 
any one species are related to each other. No natu- 
ralist pretends that all the species of a genus are equally 
distinct from each otl\er ; they may generally be divided 
into sub-genera, or sections, or lesser groups. As Fries 
has well remarked, little groups of species are generally 
clustered like satellites around certain other species. And 
what are varieties but groups of forms, unequally related 
to each other, and clustered round certain forms — that is, 
round their parent-species ? Undoubtedly there is one 
most important point of difference between varieties and 
species ; namely, that the amount of difference between 
varieties, when compared with each other or with their 
parent-species, is much less than that between the spe- 
cies of the same genus. But when we come to discuss 
the principle, as I call it, of Divergence of Character, 



we shall see how this may be explained, and how the 
lesser differences between varieties will tend to increase 
into the greater differences between species. 

There is one other point which seems to me worth 
notice. Varieties generally have much restricted ranges : 
this statement is indeed scarcely more than a truism, 
for if a variety were found to have a wider range than 
that of its supposed parent-species, their denominations 
ought to be reversed. But there is also reason to believe, 
that those species which are very closely allied to 
other species, and in so far resemble varieties, often 
have much restricted ranges. For instance, Mr. H. C. 
Watson has marked for me in the well-sifted London 
Catalogue of plants (4th edition) 63 plants which are 
therein ranked as species, but which he considers as so 
closely allied to other species as to be of doubtful value : 
these 63 reputed species range on an average over 6 '9 
of the provinces into winch Mr. Watson has divided 
Great Britain. Now, in this same catalogue, 53 acknow- 
ledged varieties are recorded, and these range over 7*7 
provinces ; whereas, the species to which these varieties 
belong range over 14*3 provinces. So that the acknow- 
ledged varieties have very nearly the same restricted 
average range, as have those very closely allied forms, 
marked for me by Mr. Watson as doubtful species, but 
which are almost universally ranked by British botanists 
as good and true species. 

Finally, then, varieties have the same general cha- 
racters as species, for they cannot be distinguished from 
species, — except, firstly, by the discovery of intermediate 
linking forms, and the occurrence of such links cannot 
affect the actual characters of the forms which they 
connect ; and except, secondly, by a certain amount of 


difference, for two forms, if differing very little, are 
generally ranked as varieties, notwithstanding that inter- 
mediate linking forms have not been discovered; but 
the amount of difference considered necessary to give 
to two forms the rank of species is quite indefinite. In 
genera having more than the average number of species 
in any country, the species of these genera have more 
than the average number of varieties. In large genera 
the species are apt to be closely, but unequally, allied 
together, forming little clusters round certain species. 
Species very closely allied to other species apparently 
have restricted ranges. In all these several respects the 
species of large genera present a strong analogy with 
varieties. And we can clearly understand these analo- 
gies, if species have once existed as varieties, and have 
thus originated : whereas, these analogies are utterly 
inexplicable if each species has been independently 

We have, also, seen that it is the most flourishing 
and dominant species of the larger genera which on an 
average vary most ; and varieties, as we shall hereafter 
see, tend to become converted into new and distinct 
species. The larger genera thus tend to become larger ; 
and throughout nature the forms of life which are now 
dominant tend to become still more dominant by leav- 
ing many modified and dominant descendants. But by 
steps hereafter to be explained, the larger genera also 
tend to break up into smaller genera. And thus, the 
forms of life throughout the universe become divided 
into groups subordinate to groups. 



Struggle for Existence. 

Bears on natural selection — The term used in a wide sense — Geo- 
metrical powers of increase — Eapid increase of naturalised 
animals and plants — Nature of the checks to increase — Compe- 
tition universal — Effects of climate — Protection from the 
number of individuals — Complex relations of all animals and 
plants throughout nature — Struggle for life most severe between 
individuals and varieties of the same species ; often severe be- 
tween species of the same genus — The relation of organism to 
organism the most important of all relations. 

Before entering on the subject of this chapter, I must 
make a few preliminary remarks, to show how the 
struggle for existence bears on Natural Selection. It 
has been seen in the last chapter that amongst organic 
beings in a state of nature there is some individual vari- 
ability ; indeed I am not aware that this has ever been 
disputed. It is immaterial for us whether a multitude 
of doubtful forms be called species or sub-species or vari- 
eties ; what rank, for instance, the two or three hundred 
doubtful forms of British plants are entitled to hold, if 
the existence of any well-marked varieties be admitted. 
But the mere existence of individual variability and of 
some few well-marked varieties, though necessary as 
the foundation for the work, helps us but little in 
understanding how species arise in nature. How have 
all those exquisite adaptations of one part of the organ- 
isation to another part, and to the conditions of life, and 
of one distinct organic being to another being, been per- 
fected? We see these beautiful co-adaptations most 
plainly in the woodpecker and missletoe ; and only a 
little less plainly in the humblest parasite which clings 


to the hairs of a quadruped or feathers of a bird ; in the 
structure of the beetle which dives through the water ; 
in the plumed seed which is wafted by the gentlest 
breeze ; in short, we see beautiful adaptations every- 
where and in every part of the organic world. 

Again, it may be asked, how is it that varieties, which 
I have called incipient species, become ultimately con- 
verted into good and distinct species, which in most 
cases obviously differ from each other far more than do 
the varieties of the same species? How do those groups 
of species, which constitute what are called distinct 
genera, and which differ from each other more than do 
the species of the same genus, arise ? All these results, 
as we shall more fully see in the next chapter, follow 
inevitably from the struggle for life. Owing to this 
struggle for life, any variation, however slight and from 
whatever cause proceeding, if it be in any degree pro- 
fitable to an individual of any species, in its infinitely 
complex relations to other organic beings and to ex- 
ternal nature, will tend to the preservation of that indi- 
vidual, and will generally be inherited by its offspring. 
The offspring, also, will thus have a better chance of 
surviving, for, of the many individuals of any species 
winch are periodically born, but a small number can 
survive. I have called tins principle, by which each 
slight variation, if useful, is preserved, by the term of 
Natural Selection, in order to mark its relation to man's 
power of selection. We have seen that man by selec- 
tion can certainly produce great results, and can adapt 
organic beings to his own uses, through the accumula- 
tion of slight but useful variations, given to him by the 
hand of Nature. But Natural Selection, as we shall 
hereafter see, is a power incessantly ready for action, 
and is as immeasurably superior to man's feeble efforts, 
as the works of Nature are to those of Art. 


We will now discuss in a little more detail the struggle 
for existence. In my future work this subject shall be 
treated, as it well deserves, at much greater length. 
The elder De Candolle and Lyell have largely and phi- 
losophically shown that all organic beings are exposed 
to severe competition. In regard to plants, no one has 
treated this subject with more spirit and ability than 
W. Herbert, Dean of Manchester, evidently the result 
of his great horticultural knowledge. Nothing is easier 
than to admit in words the truth of the universal 
struggle for life, or more difficult — at least I have found 
it so — than constantly to bear this conclusion in mind. 
Yet unless it be thoroughly engrained in the mind, I 
am convinced that the whole economy of nature, with 
every fact on distribution, rarity, abundance, extinction, 
and variation, will be dimly seen or quite misunderstood. 
We behold the face of nature bright with gladness, we 
often see superabundance of food ; we do not see, or we 
forget, that the birds which are idly singing round us 
mostly live on insects or seeds, and are thus constantly 
destroying life ; or we forget how largely these songsters, 
or their eggs, or their nestlings, are destroyed by birds 
and beasts of prey ; we do not always bear in mind, 
that though food may be now superabundant, it is not 
so at all seasons of each recurring year. 

I should premise that I use the term Struggle for 
Existence in a large and metaphorical sense, including 
dependence of one being on another, and including 
(which is more important) not only the life of the indi- 
vidual, but success in leaving progeny. Two canine 
animals in a time of dearth, may be truly said to 
struggle with each other which shall get food and live. 
But a plant on the edge of a desert is said to struggle 
for life against the drought, though more properly it 
should be said to be dependent on the moisture. A 


plant which annually produces a thousand seeds, of 
winch on an average only one comes to maturity, may - 
be more truly said to struggle with the plants of the 
same and other kinds which already clothe the ground. 
The missletoe is dependent on the apple and a few other 
trees, but can only in a far-fetched sense be said to 
struggle with these trees, for if too many of these para- 
sites grow on the same tree, it will languish and die. 
But several seedling missletoes, growing close together 
on the same branch, may more truly be said to struggle 
with each other. As the missletoe is disseminated by 
birds, its existence depends on birds ; and it may meta- 
phorically be said to struggle with other fruit-bearing 
plants, in order to tempt birds to devour and thus 
disseminate its seeds rather than those of other plants. 
In these several senses, which pass into each other, I 
use for convenience sake the general term of struggle 
for existence. 

A struggle for existence inevitably follows from the 
high rate at which all organic beings tend to increase. 
Every being, which during its natural lifetime produces 
several eggs or seeds, must suffer destruction during some 
period of its life, and during some season or occasional 
year, otherwise, on the principle of geometrical increase, 
its numbers would quickly become so inordinately great 
that no country could support the product. Hence, as 
more individuals are produced than can possibly sur- 
vive, there must in every case be a struggle for exist- 
ence, either one individual with another of the same 
species, or with the individuals of distinct species, or 
with the physical conditions of life. It is the doctrine 
of Malthus applied with manifold force to the whole 
animal and vegetable kingdoms ; for in this case there 
can be no artificial increase of food, and no prudential 
restraint from marriage. Although some species may 


be now increasing, more or less rapidly, in numbers, all 
cannot do so, for the world would not hold them. 

There is no exception to the rule that every organic 
being naturally increases at so high a rate, that if not 
destroyed, the earth would soon be covered by the progeny 
of a single pair. Even slow-breeding man has doubled 
in twenty-five years, and at this rate, in a few thousand 
years, there would literally not be standing room for his 
progeny. Linnaeus has calculated that if an annual 
plant produced only two seeds — and there is no plant so 
unproductive as this — and their seedlings next year pro- 
duced two, and so on, then in twenty years there would 
be a million plants. The elephant is reckoned to be the 
slowest breeder of all known animals, and I have taken 
some pains to estimate its probable minimum rate of 
natural increase : it will be under the mark to assume 
that it breeds when thirty years old, and goes on breeding 
till ninety years old, bringing forth three pair of young 
in this interval ; if this be so, at the end of the fifth 
century there would be alive fifteen million elephants, 
descended from the first pair. 

But we have better evidence on this subject than 
mere theoretical calculations, namely, the numerous 
recorded cases of the astonishingly rapid increase of 
various animals in a state of nature, when circumstances 
have been favourable to them during two or three fol- 
lowing seasons. Still more striking is the evidence from 
our domestic animals of many kinds which have run 
wild in several parts of the world : if the statements of 
the rate of increase of slow-breeding cattle and horses 
in South-America, and latterly in Australia, had not 
been well authenticated, they would have been quite 
incredible. So it is with plants : cases could be given of 
introduced plants which have become common throughout 
whole islands in a period of less than ten years. Several 


of the plants now most numerous over the wide plains of 
La Plata, clothing square leagues of surface almost to 
the exclusion of all other plants, have been introduced 
from Europe ; and there are plants winch now range in 
India, as I hear from Dr. Falconer, from Cape Comorin 
to the Himalaya, which have been imported from America 
since its discovery. In such cases, and endless instances 
could be given, no one supposes that the fertility of 
these animals or plants has been suddenly and tempo- 
rarily increased in any sensible degree. The obvious 
explanation is that the conditions of life have been very 
favourable, and that there has consequently been less 
destruction of the old and young, and that nearly all the 
young have been enabled to breed. In such cases the 
geometrical ratio of increase, the result of which never 
fails to be surprising, simply explains the extraordinarily 
rapid increase and wide diffusion of naturalised produc- 
tions in their new homes. 

In a state of nature almost every plant produces seed, 
and amongst animals there are very few which do not 
annually pair. Hence we may confidently assert, that 
all plants and animals are tending to increase at a geo- 
metrical ratio, that all would most rapidly stock every 
station in which they could any how exist, and that the 
geometrical tendency to increase must be checked by 
destruction at some period of life. Our familiarity with 
the larger domestic animals tends, I think, to mislead 
us : we see no great destruction falling on them, and we 
forget that thousands are annually slaughtered for food, 
and that in a state of nature an equal number would 
have somehow to be disposed of. 

The only difference between organisms which annually 
produce eggs or seeds by the thousand, and those which 
produce extremely few, is, that the slow-breeders would 
require a few more years to people, under favourable 


conditions, a whole district, let it be ever so large. The 
condor lays a couple of eggs and the ostrich a score, and 
yet in the same country the condor may be the more 
numerous of the two : the Fulmar petrel lays but one 
egg, yet it is believed to be the most numerous bird 
in the world. One fly deposits hundreds of eggs, and 
another, like the hippobosca, a single one ; but this 
difference does not determine how many individuals of 
the two species can be supported in a district. A large 
number of eggs is of some importance to those species, 
which depend on a rapidly fluctuating amomit of food, 
for it allows them rapidly to increase in number. But 
the real importance of a large number of eggs or seeds 
is to make up for much destruction at some period of 
life ; and this period in the great majority of cases is an 
early one. If an animal can in any way protect its own 
eggs or young, a small number may be produced, and 
yet the average stock be fully kept up; but if many 
eggs or young are destroyed, many must be produced, 
or the species will become extinct. It would suffice to 
keep up the full number of a tree, which lived on an 
average for a thousand years, if a single seed were pro- 
duced once in a thousand years, supposing that this seed 
were never destroyed, and could be ensured to germi- 
nate in a fitting place. So that in all cases, the average 
number of any animal or plant depends only indirectly 
on the number of its eggs or seeds. 

In looking at Nature, it is most necessary to keep 
the foregoing considerations always in mind — never to 
forget that every single organic being around us may 
be said to be striving to the utmost to increase in num- 
bers ; that each lives by a struggle at some period of 
its life; that heavy destruction inevitably falls either 
on the young or old, during each generation or at 
recurrent intervals. Lighten any check, mitigate the 


destruction ever so little, and the number of the species 
will almost instantaneously increase to any amount. 
The face of Nature may be compared to a yielding 
surface, with ten thousand sharp wedges packed close 
together and driven inwards by incessant blows, some- 
times one wedge being struck, and then another with 
greater force. 

What checks the natural tendency of each species to 
increase in number is most obscure. Look at the most 
vigorous species ; by as much as it swarms in numbers, by 
so much will its tendency to increase be still further in- 
creased. We know not exactly what the checks are in 
even one single instance. Nor will this surprise any one 
who reflects how ignorant we are on this head, even in 
regard to mankind, so incomparably better known than 
any other animal. This subject has been ably treated by 
several authors, and I shall, in my future work, discuss 
some of the checks at considerable length, more especially 
in regard to the feral animals of South America. Here 
I will make only a few remarks, just to recall to the 
reader's mind some of the chief points. Eggs or very 
young animals seem generally to suffer most, but this 
is not invariably the case. With plants there is a vast 
destruction of seeds, but, from some observations which 
I have made, I believe that it is the seedlings winch 
suffer most from germinating in ground already thickly 
stocked with other plants. Seedlings, also, are destroyed 
in vast numbers by various enemies ; for instance, on a 
piece of ground three feet long and two wide, dug and 
cleared, and where there could be no choking from other 
plants, I marked all the seedlings of our native weeds as 
they came up, and out of the 357 no less than 295 were 
destroyed, chiefly by slugs and insects. If turf which 
has long been mown, and the case would be the same 
with turf closely browsed by quadrupeds, be let to grow, 


the more vigorous plants gradually kill the less vigorous, 
though fully grown, plants : thus out of twenty species 
growing on a little plot of turf (three feet by four) nine 
species perished from the other species being allowed to 
grow up freely. 

The amount of food for each species of course gives 
the extreme limit to which each can increase ; but very 
frequently it is not the obtaining food, but the serving 
as prey to other animals, which determines the average 
numbers of a species. Thus, there seems to be little 
doubt that the stock of partridges, grouse, and hares on 
any large estate depends chiefly on the destruction of 
vermin. If not one head of game were shot during the 
next twenty years in England, and, at the same time, 
if no vermin were destroyed, there would, in all proba- 
bility, be less game than at present, although hundreds 
of thousands of game animals are now T annually killed. 
On the other hand, in some cases, as with the elephant 
and rhinoceros, none are destroyed by beasts of prey : 
even the tiger in India most rarely dares to attack a 
young elephant protected by its dam. 

Climate plays an important part in determining the 
average numbers of a species, and periodical seasons of 
extreme cold or drought, I believe to be the most 
effective of all checks. I estimated that the winter of 
1854-55 destroyed four-fifths of the birds in my own 
grounds ; and this is a tremendous destruction, when we 
remember that ten per cent, is an extraordinarily severe 
mortality from epidemics with man. The action of cli- 
mate seems at first sight to be quite independent of the 
struggle for existence ; but in so far as climate chiefly 
acts in reducing food, it brings on the most severe 
struggle between the individuals, whether of the same 
or of distinct species, which subsist on the same kind 
of food. Even when climate, for instance extreme 


cold, acts directly, it will be the least vigorous, or those 
which have got least food through the advancing winter, 
which will suffer most. When we travel from south to 
north, or from a damp region to a dry, we invariably 
see some species gradually getting rarer and rarer, and 
finally disappearing ; and the change of climate being 
conspicuous, we are tempted to attribute the whole 
effect to its direct action. But this is a very false view : 
we forget that each species, even where it most abounds, 
is constantly suffering enormous destruction at some 
period of its life, from enemies or from competitors for 
the same place and food ; and if these enemies or com- 
petitors be in the least degree favoured by any slight 
change of climate, they will increase in numbers, and, 
as each area is already fully stocked with inhabitants, 
the other species will decrease. When we travel south- 
ward and see a species decreasing in numbers, we may 
feel sure that the cause lies quite as much in other spe- 
cies being favoured, as in this one being hurt. So it is 
when we travel northward, but in a somewhat lesser 
degree, for the number of species of all kinds, and there- 
fore of competitors, decreases northwards ; hence in 
going northward, or in ascending a mountain, we far 
oftener meet with stunted forms, due to the directly 
injurious action of climate, than we do in proceeding 
southwards or in descending a mountain. When we 
reach the Arctic regions, or snow-capped summits, or 
absolute deserts, the struggle for life is almost exclu- 
sively with the elements. 

That climate acts in main part indirectly by favouring 
other species, we may clearly see in the prodigious 
number of plants in our gardens which can perfectly 
well endure our climate, but which never become natu- 
ralised, for they cannot compete with our native plants, 
nor resist destruction by our native animals. 


When a species, owing to highly favourable circum- 
stances, increases inordinately in numbers in a small 
tract, epidemics — at least, this seems generally to occur 
with our game animals — often ensue : and here we have 
a limiting check independent of the struggle for life. 
But even some of these so-called epidemics appear to 
be due to parasitic worms, which have from some cause, 
possibly in part through facility of diffusion amongst 
the crowded animals, .been disproportionably favoured : 
and here comes in a sort of struggle between the para- 
site and its prey. 

On the other hand, in many cases, a large stock of 
individuals of the same species, relatively to the num- 
bers of its enemies, is absolutely necessary for its pre- 
servation. Thus we can easily raise plenty of corn and 
rape-seed, &c, in our fields, because the seeds are in 
great excess compared with the number of birds which 
feed on them ; nor can the birds, though having a super- 
abundance of food at this one season, increase in number 
proportionally to the supply of seed, as their numbers 
are checked during winter : but any one who has tried, 
knows how troublesome it is to get seed from a few 
wheat or other such plants in a garden ; I have in this 
case lost every single seed. This view of the necessity 
of a large stock of the same species for its preservation, 
explains, I believe, some singular facts in nature, such 
as that of very rare plants being sometimes extremely 
abundant in the few spots where they do occur; and 
that of some social plants being social, that is, abound- 
ing in individuals, even on the extreme confines of their 
range. For in such cases, we may believe, that a plant 
could exist only where the conditions of its life were so 
favourable that many could exist together, and thus save 
each other from utter destruction. I should add that tin s 
good effects of frequent intercrossing, and the ill effects 


of close interbreeding, probably come into play in some 
of these cases ; but on this intricate subject I will not 
here enlarge. 

Many cases are on record showing how complex 
and unexpected are the checks and relations between 
organic beings, which have to struggle together in the 
same country. I will give only a single instance, which, 
though a simple one, has interested me. In Stafford- 
shire, on the estate of a relation where I had ample 
means of investigation, there was a large and extremely 
barren heath, which had never been touched by the 
hand of man ; but several hundred acres of exactly the 
same nature had been enclosed twenty-five years pre- 
viously and planted with Scotch fir. The change in the 
native vegetation of the planted part of the heath was 
most remarkable, more than is generally seen in passing 
from one quite different soil to another : not only the 
proportional numbers of the heath-plants were wholly 
changed, but twelve species of plants (not counting 
grasses and carices) floimshed in the plantations, which 
could not be found on the heath. The effect on the 
insects must have been still greater, for six insectivorous 
birds were very common in the plantations, which were 
not to be seen on the heath ; and the heath was fre- 
quented by two or three distinct insectivorous birds. 
Here we see how potent has been the effect of the in- 
troduction of a single tree, nothing whatever else having 
been done, with the exception that the land had been 
enclosed, so that cattle could not enter. But how im- 
portant an element enclosure is, I plainly saw near 
Farnham, in Surrey. Here there are extensive heaths, 
with a few clumps of old Scotch firs on the distant hill- 
tops : within the last ten years large spaces have been 
enclosed, and self-sown firs are now springing up in 
multitudes, so close together that all cannot live. 


When I ascertained that these young trees had not 
been sown or planted, I was so much surprised at their 
numbers that I went to several points of view, whence 
I could examine hundreds of acres of the unenclosed 
heath, and literally I could not see a single Scotch fir, 
except the old planted clumps. But on looking closely 
between the stems of the heath, I found a multitude of 
seedlings and little trees, which had been perpetually 
browsed down by the cattle. In one square yard, at a 
point some hundred yards distant from one of the old 
clumps, I counted thirty-two little trees ; and one of them, 
judging from the rings of growth, had during twenty-six 
years tried to raise its head above the stems of the heath, 
and had failed. No wonder that, as soon as the land 
was enclosed, it became thickly clothed with vigorously 
growing young firs. Yet the heath was so extremely 
barren and so extensive that no one would ever have 
imagined that cattle would have so closely and effectu- 
ally searched it for food. 

Here we see that cattle absolutely determine the 
existence of the Scotch fir ; but in several parts of the 
world insects determine the existence of cattle. Perhaps 
Paraguay offers the most curious instance of this ; for 
here neither cattle nor horses nor dogs have ever run 
wild, though they swarm southward and northward in a 
feral state ; and Azara and Eengger have shown that 
this is caused by the greater number in Paraguay of a 
certain fly, which lays its eggs in the navels of these 
animals when first born. The increase of these flies, 
numerous as they are, must be habitually checked by 
some means, probably by birds. Hence, if certain in- 
sectivorous birds (whose numbers are probably regulated 
by hawks or beasts of prey) were to increase in Paraguay, 
the flies would decrease — then cattle and horses would 
become feral, and this would certainly greatly alter (as 


indeed I have observed in parts of South America) the 
vegetation : this again would largely affect the insects ; 
and this, as we just have seen in Staffordshire, the 
insectivorous birds, and so onwards in ever-increasing 
circles of complexity. We began this series by insecti- 
vorous birds, and we have ended with them. Not that 
in nature the relations can ever be as simple as this. 
Battle within battle must ever be recurring with varying 
success; and yet in the long-run the forces are so 
nicely balanced, that the face of nature remains uniform 
for long periods of time, though assuredly the merest 
trifle would often give the victory to one organic being 
over another. Nevertheless so profound is our ignorance, 
and so high our presumption, that we marvel when we 
hear of the extinction of an organic being ; and as we do 
not see the cause, we invoke cataclysms to desolate the 
world, or invent laws on the duration of the forms of life ! 
I am tempted to give one more instance showing how 
plants and animals, most remote in the scale of nature, 
are bound together by a web of complex relations. I 
shall hereafter have occasion to show that the exotic 
Lobelia fulgens, in this part of England, is never visited 
by insects, and consequently, from its peculiar structure, 
never can set a seed. Many of our orchidaceous plants 
absolutely require the visits of moths to remove their 
pollen-masses and thus to fertilise them. I have, also, 
reason to believe that humble-bees are indispensable to 
the fertilisation of the heartsease (Viola tricolor), for 
other bees do not visit this flower. From experiments 
which I have tried, I have found that the visits of bees, 
if not indispensable, are at least highly beneficial to the 
fertilisation of our clovers; but humble-bees alone visit 
the common red clover (Trifolium pratense), as other 
bees cannot reach the nectar. Hence I have very little 
doubt, that if the whole genus of humble-bees became 


extinct or very rare in England, the heartsease and red 
clover would become very rare, or wholly disappear. 
The number of humble-bees in any district depends in 
a great degree on the number of field-mice, which de- 
stroy their combs and nests ; and Mr. H. Newman, who 
has long attended to the habits of humble-bees, believes 
that " more than two-thirds of them are thus destroyed 
all over England." Now the number of mice is largely 
dependent, as every one knows, on the number of cats ; 
and Mr. Newman says, " Near villages and small towns 
I have found the nests of humble-bees more numerous 
than elsewhere, which I attribute to the number of cats 
that destroy the mice." Hence it is quite credible that 
the presence of a feline animal in large numbers in a 
district might determine, through the intervention first 
of mice and then of bees, the frequency of certain 
flowers in that district ! 

In the case of every species, many different checks, 
acting at different periods of life, and during different 
seasons or years, probably come into play; some one 
check or some few being generally the most potent, but 
all concurring in determining the average number or 
even the existence of the species. In some cases it can 
be shown that widely-different checks act on the same 
species in different districts. When we look at the 
plants and bushes clothing an entangled bank, we are 
tempted to attribute their proportional numbers and 
kinds to what we call chance. But how false a view 
is this ! Every one has heard that when an American 
forest is cut down, a very different vegetation springs 
up; but it has been observed that the trees now 
growing on the ancient Indian mounds, in the Southern 
United States, display the same beautiful diversity and 
proportion of kinds as in the surrounding virgin forests. 
What a struggle between the several kinds of trees 


must here have gone on during long centuries, each an- 
nually scattering its seeds by the thousand ; what war 
between insect and insect — between insects, snails, and 
other animals with birds and beasts of prey — all striving 
to increase, and all feeding on each other or on the trees 
or their seeds and seedlings, or on the other plants which 
first clothed the ground and thus checked the growth of 
the trees ! Throw up a handful of feathers, and all must 
fall to the ground according to definite laws ; but how 
simple is this problem compared to the action and re- 
action of the innumerable plants and animals which have 
determined, in the course of centuries, the proportional 
numbers and kinds of trees now growing on the old 
Indian ruins ! 

The dependency of one organic being on another, as 
of a parasite on its prey, lies generally between beings 
remote in the scale of nature. This is often the case 
with those which may strictly be said to struggle with 
each other for existence, as hi the case of locusts and 
grass-feeding quadrupeds; But the struggle almost in- 
variably will be most severe between the individuals of 
the same species, for they frequent the same districts, 
require the same food, and are exposed to the same 
dangers. In the case of varieties of the same species, 
the struggle will generally be almost equally severe, 
and we sometimes see the contest soon decided: for 
instance, if several varieties of wheat be sown together, 
and the mixed seed be resown, some of the varieties 
which best suit the soil or climate, or are naturally the 
most fertile, will beat the others and so yield more 
seed, and will consequently in a few years quite sup- 
plant the other varieties. To keep up a mixed stock 
of even such extremely close varieties as the variously 
coloured sweet-peas, they must be each year harvested 
separately, and the seed then mixed in due propor- 



tion, otherwise the weaker kinds will steadily decrease 
in numbers and disappear. So again with the varieties 
of sheep : it has been asserted that certain mountain- 
varieties will starve out other mountain-varieties, so 
that they cannot be kept together. The same result 
has followed from keeping together different varieties 
of the medicinal leech. It may even be doubted whe- 
ther the varieties of any one of our domestic plants 
or animals have so exactly the same strength, habits, 
and constitution, that the original proportions of a 
mixed stock could be kept up for half a dozen genera- 
tions, if they were allowed to struggle together, like 
beings in a state of nature, and if the seed or young 
were hot annually sorted. 

As species of the same genus have usually, though 
by no means invariably, some similarity in habits and 
constitution, and always in structure, the struggle will 
generally be more severe between species of the same 
genus, when they come into competition with each other, 
than between species of distinct genera. We see this in 
the recent extension over parts of the United States of 
one species of swallow having caused the decrease of an- 
other species. The recent increase of the missel-thrush 
in parts of Scotland has caused the decrease of the 
song-thrush. How frequently we hear of one species 
of rat taking the place of another species under the 
most different climates! In Eussia the small Asiatic 
cockroach has everywhere driven before it its great 
congener. One species of charlock will supplant another, 
and so in other cases. We can dimly see why the com- 
petition should be most severe between allied forms, 
which fill nearly the same place in the economy of nature ; 
but probably in no one case could we precisely say why 
one species has been victorious over another in the 
great battle of life. 


A corollary of the highest importance may be deduced 
from the foregoing remarks, namely, that the structure 
of every organic being is related, in the most essential 
yet often hidden manner, to that of all other organic 
beings, with which it comes into competition for food or 
residence, or from which it has to escape, or on which it 
preys. This is obvious in the structure of the teeth and 
talons of the tiger ; and in that of the legs and claws of 
the parasite which clings to the hair on the tiger's body. 
But in the beautifully plumed seed of the dandelion, 
and in the flattened and fringed legs of the water-beetle, 
the relation seems at first confined to the elements of 
air and water. Yet the advantage of plumed seeds no 
doubt stands in the closest relation to the land being 
already thickly clothed by other plants; so that the 
seeds may be widely distributed and fall on unoc- 
cupied ground. In the water-beetle, the structure of 
its legs, so well adapted for diving, allows it to 
compete with other aquatic insects, to hunt for 
its own prey, and to escape serving as prey to other 

The store of nutriment laid up within the seeds of 
many plants seems at first sight to have no sort of 
relation to other plants. But from the strong growth 
of young plants produced from such seeds (as peas 
and beans), when sown in the midst of long grass, 
I suspect that the chief use of the nutriment in the 
seed is to favour the growth of the young seedling, 
whilst struggling with other plants growing vigorously 
all around. 

Look at a plant in the midst of its range, why does 
it not double or quadruple its numbers? We know 
that it can perfectly well withstand a little more heat 
or cold, dampness or dryness, for elsewhere it ranges 


into slightly hotter or colder, damper or drier districts. 
In this case we can clearly see that if we wished in 
imagination to give the plant the power of increasing 
in number, we should have to give it some advantage 
over its competitors, or over the animals which preyed 
on it. On the confines of its geographical range, a change 
of constitution with respect to climate would clearly 
be an advantage to our plant; but we have reason 
to believe that only a few plants or animals range so 
far, that they are destroyed by the rigour of the climate 
alone. Not until we reach the extreme confines of life, 
in the arctic regions or on the borders of an utter desert, 
will competition cease. The land may be extremely 
cold or dry, yet there will be competition between some 
few species, or between the individuals of the same 
species, for the warmest or dampest spots. 

Hence, also, we can see that when a plant or animal 
is placed in a new country amongst new competitors, 
though the climate may be exactly the same as in its 
former home, yet the conditions of its life will generally 
be changed in an essential manner. If we wished to in- 
crease its average numbers in its new home, we should 
have to modify it in a different way to what we should 
have done in its native country ; for we should have to 
give it some advantage over a different set of com- 
petitors or enemies. 

It is good thus to try in our imagination to give any 
form some advantage over another. Probably in no 
single instance should we know what to do, so as to 
succeed. It will convince us of our ignorance on the 
mutual relations of all organic beings ; a conviction as 
necessary, as it seems to be difficult to acquire. All 
that we can do, is to keep steadily in mind that each 
organic being is striving to increase at a geometrical 


ratio ; that each at some period of its life, during some 
season of the year, during each generation or at in- 
tervals, has to struggle for life, and to suffer great de- 
struction. When we reflect on this struggle, we may- 
console ourselves with the full belief, that the war of 
nature is not incessant, that no fear is felt, that death 
is generally prompt, and that the vigorous, the healthy, 
and the happy survive and multiply. 



Natural Selection. 

Natural Selection — its power compared with man's selection — its 
power on characters of trifling importance — its power at all ages 
and on "both sexes — Sexual Selection — On the generality of inter- 
crosses between individuals of the same species — Circumstances 
favourable and unfavourable to Natural Selection, namely, 
intercrossing, isolation, number of individuals — Slow action — 
Extinction caused by Natural Selection — Divergence of Cha- 
racter, related to the diversity of inhabitants of any small area, 
and to naturalisation — Action of Natural Selection, through 
Divergence of Character and Extinction, on the descendants from 
a common parent — Explains the Grouping of all organic beings. 

How will the struggle for existence, discussed too briefly 
in the last chapter, act in regard to variation ? Can 
the principle of selection, which we have seen is so 
potent in the hands of man, apply in nature ? I think 
we shall see that it can act most effectually. Let it be 
borne in mind in what an endless number of strange 
peculiarities our domestic productions, and, in a lesser 
degree, those under nature, vary ; and how strong the 
hereditary tendency is. Under domestication, it may be 
truly said that the whole organisation becomes in some 
degree plastic. Let it be borne in mind how infinitely 
complex and close-fitting are the mutual relations of all 
organic beings to each other and to their physical con- 
ditions of life. Can it, then, be thought improbable, 
seeing that variations useful to man have undoubtedly 
occurred, that other variations useful in some way to 
each being in the great and complex battle of life, 
should sometimes occur in the course of thousands of 
generations ? If such do occur, can we doubt (remem- 


bering that many more individuals are born than can 
possibly survive) that individuals having any advantage, 
however slight, over others, would have the best 
chance of surviving and of procreating their kind? 
On the other hand, we may feel sure that any varia- 
tion in the least degree injurious would be rigidly 
destroyed. This preservation of favourable variations 
and the rejection of injurious variations, I call Natural 
Selection. Variations neither useful nor injurious would 
not be affected by natural selection, and would be left 
a fluctuating element, as perhaps we see in the species 
called polymorphic. 

We shall best understand the probable course of 
natural selection by taking the case of a country under- 
going some physical change, for instance, of climate. 
The proportional numbers of its inhabitants would 
almost immediately undergo a change, and some species 
might become extinct. We may conclude, from what 
we have seen of the intimate and complex manner in 
which the inhabitants of each country are bound to- 
gether, that any change in the numerical proportions of 
some of the inhabitants, independently of the change 
of climate itself, would most seriously affect many of 
the others. If the country were open on its borders, 
new forms would certainly immigrate, and this also 
would seriously disturb the relations of some of the 
former inhabitants. Let it be remembered how powerful 
the influence of a single introduced tree or mammal has 
been shown to be. But in the case of an island, or of 
a country partly surrounded by barriers, into which new 
and better adapted forms could not freely enter, we 
should then have places in the economy of nature which 
would assuredly be better filled up, if some of the ori- 
ginal inhabitants were in some manner modified; for, 
had the area been open to immigration, these same 

E 3 


places would have been seized on by intruders. In such 
case, every slight modification, which in the course of 
ages chanced to arise, and which in any way favoured 
the individuals of any of the species, by better adapting 
them to their altered conditions, would tend to be pre- 
served ; and natural selection would thus have free 
scope for the work of improvement. 

We have reason to believe, as stated in the first 
chapter, that a change in the conditions of life, by 
specially acting on the reproductive system, causes or 
increases variability; and in the foregoing case the 
conditions of life are supposed to have undergone a 
change, and this would manifestly be favourable to 
natural selection, by giving a better chance of profitable 
variations occurring ; and unless profitable variations do 
occur, natural selection can do nothing. Not that, as I 
believe, any extreme amount of variability is necessary ; 
as man can certainly produce great results by adding 
up in any given direction mere individual differences, 
so could Nature, but far more easily, from having incom- 
parably longer time at her disposal. Nor do I believe 
that any great physical change, as of climate, or any 
unusual degree of isolation to check immigration, is 
actually necessary to produce new and unoccupied 
places for natural selection to fill up by modifying 
and improving some of the varying inhabitants. For 
as all the inhabitants of each country are struggling 
together with nicely balanced forces, extremely slight 
modifications in the structure or habits of one inha- 
bitant would often give it an advantage over others; 
and still further modifications of the same kind would 
often still further increase the advantage. No country 
can be named in which all the native inhabitants are 
now so perfectly adapted to each other and to the 
physical conditions under which they live, that none of 


them could anyhow be improved ; for in all countries, 
the natives have been so far conquered by naturalised 
productions, that they have allowed foreigners to take 
firm possession of the land. And as foreigners have 
thus everywhere beaten some of the natives, we may 
safely conclude that the natives might have been mo- 
dified with advantage, so as to have better resisted such 

As man can produce and certainly has produced a 
great result by his methodical and unconscious means 
of selection, what may not nature effect ? Man can act 
only on external and visible characters: nature cares 
nothing for appearances, except in so far as they may 
be useful to any being. She can act on every internal 
organ, on every shade of constitutional difference, on the 
whole machinery of life. Man selects only for his own 
good ; Nature only for that of the being which she tends. 
Every selected character is fully exercised by her ; and 
the being is placed under well-suited conditions of life. 
Man keeps the natives of many climates in the same 
country; he seldom exercises each selected character 
in some peculiar and fitting manner; he feeds a long 
and a short beaked pigeon on the same food; he 
does not exercise a long-backed or long-legged qua- 
druped in any peculiar manner ; he exposes sheep 
with long and short wool to the same climate. He 
does not allow the most vigorous males to struggle 
for the females. He does not rigidly destroy all in- 
ferior animals, but protects during each varying season, 
as far as lies in his power, all his productions. He 
often begins his selection by some half-monstrous form ; 
or at least by some modification prominent enough to 
catch his eye, or to be plainly useful to him. Under 
nature, the slightest difference of structure or consti- 
tution may well turn the nicely-balanced scale in the 


struggle for life, and so be preserved. How fleeting are 
the wishes and efforts of man ! how short his time ! and 
consequently how poor will his products be, compared 
with those accumulated by nature during whole geolo- 
gical periods. Can we wonder, then, that nature's pro- 
ductions should be far " truer " in character than man's 
productions ; that they should be infinitely better 
adapted to the most complex conditions of life, and 
should plainly bear the stamp of far higher workman- 
ship ? 

It may be said that natural selection is daily and 
hourly scrutinising, throughout the world, every varia- 
tion, even the slightest ; rejecting that which is bad, 
preserving and adding up all that is good ; silently and 
insensibly working, whenever and wherever opportu- 
nity offers, at the improvement of each organic being 
in relation to its organic and inorganic conditions cf 
life. We see nothing of these slow changes in progress, 
until the hand of time has marked the long lapse of 
ages, and then so imperfect is our view into long past 
geological ages, that we only see that the forms of life 
are now different from what they formerly were. 

Although natural selection can act only through and 
for the good of each being, yet characters and structures, 
which we are apt to consider as of very trifling import- 
ance, may thus be acted on. When we see leaf-eating 
insects green, and bark-feeders mottled-grey ; the 
alpine ptarmigan white in winter, the red-grouse the 
colour of heather, and the black-grouse that of peaty 
earth, we must believe that these tints are of service to 
these birds and insects in preserving them from danger. 
Grouse, if not destroyed at some period of their lives, 
would increase in countless numbers ; they are known 
to suffer largely from birds of prey; and hawks are 
guided by eyesight to their prey, — so much so, that on 


parts of the Continent persons are warned not to keep 
white pigeons, as being the most liable to destruction. 
Hence I can see no reason to doubt that natural selec- 
tion might be most effective in giving the proper colour 
to each kind of grouse, and in keeping that colour, 
when once acquired, true and constant. Nor ought we 
to think that the occasional destruction of an animal of 
any particular colour would produce little effect : we 
should remember how essential it is in a flock of white 
sheep to destroy every lamb with the faintest trace of 
black. In plants the down on the fruit and the colour 
of the flesh are considered by botanists as characters of 
the most trifling importance : yet we hear from an 
excellent horticulturist, Downing, that in the United 
States smooth-skinned fruits suffer far more from a 
beetle, a curculio, than those with down ; that purple 
plums suffer far more from a certain disease than yellow 
plums ; whereas another disease attacks yellow-fleshed 
peaches far more than those with other coloured flesh. 
If, with all the aids of art, these slight differences make 
a great difference in cultivating the several varieties, 
assuredly, in a state of nature, where the trees would 
have to struggle with other trees and with a host of 
enemies, such differences would effectually settle which 
variety, whether a smooth or downy, a yellow or purple 
fleshed fruit, should succeed. 

In looking at many small points of difference between 
species, which, as far as our ignorance permits us to 
judge, seem to be quite unimportant, we must not forget 
that climate, food, &c, probably produce some slight 
and direct effect. It is, however, far more necessary 
to bear in mind that there are many unknown laws of 
correlation of growth, which, when one part of the 
organisation is modified through variation, and the 
modifications arc accumulated by natural selection for 



the good of the being, will cause other modifications, 
often of the most unexpected nature. 

As we see that those variations which under domesti- 
cation appear at any particular period of life, tend to 
reappear in the offspring at the same period; — for in- 
stance, in the seeds of the many varieties of our culinary 
and agricultural plants; in the caterpillar and cocoon 
stages of the varieties of the silkworm ; in the eggs of 
poultry, and in the colour of the down of their chickens ; 
in the horns of our sheep and cattle when nearly adult ; — 
so in a state of nature, natural selection will be enabled 
to act on and modify organic beings at any age, by the 
accumulation of profitable variations at that age, and by 
their inheritance at a corresponding age. If it profit a 
plant to have its seeds more and more widely dissemi- 
nated by the wind, I can see no greater difficulty in this 
being effected through natural selection, than in the 
cotton-planter increasing and improving by selection 
the down in the pods on his cotton-trees. Natural 
selection may modify and adapt the larva of an insect 
to a score of contingencies, wholly different from those 
which concern the mature insect. These modifications 
will no doubt affect, through the laws of correlation, the 
structure of the adult ; and probably in the case of those 
insects which live only for a few hours, and which never 
feed, a large part of their structure is merely the cor- 
related result of successive changes in the structure of 
their larvse. So, conversely, modifications in the adult 
will probably often affect the structure of the larva ; but 
in all cases natural selection will ensure that modifica- 
tions consequent on other modifications at a different 
period of life, shall not be in the least degree injurious : 
for if they became so, they would cause the extinction 
of the species. 

Natural selection will modify the structure of the 


young in relation to the parent, and of the parent in 
relation to the young. In social animals it will adapt 
the structure of each individual for the benefit of the 
community ; if each in consequence profits by the selected 
change. What natural selection cannot do, is to modify 
the structure of one species, without giving it any 
advantage, for the good of another species ; and though 
statements to this effect may be found in works of 
natural history, I cannot find one case which will bear 
investigation. A structure used only once in an animal's 
whole life, if of high importance to it, might be modi- 
fied to any extent by natural selection ; for instance, 
the great jaws possessed by certain insects, and used 
exclusively for opening the cocoon — or the hard tip to 
the beak of nestling birds, used for breaking the egg. 
It has been asserted, that of the best short-beaked 
tumbler-pigeons more perish in the egg than are able to 
get out of it ; so that fanciers assist in the act of hatch- 
ing. Now, if nature had to make the beak of a full- 
grown pigeon very short for the bird's own advantage, the 
process of modification would be very slow, and there 
would be simultaneously the most rigorous selection of 
the young birds within the egg, which had the most pow- 
erful and hardest beaks, for all with weak beaks would 
inevitably perish : or, more delicate and more easily 
broken shells might be selected, the thickness of the 
shell being known to vary like every other structure. 

Sexual Selection. — Inasmuch as peculiarities often 
appear under domestication in one sex and become 
hereditarily attached to that sex, the same fact pro- 
bably occurs under nature, and if so, natural selection 
will be able to modify one sex in its functional relations 
to the other sex, or in relation to wholly different 
habits of life in the two sexes, as is sometimes the 


with insects. And this leads me to say a few words on 
what I call Sexual Selection. This depends, not on a 
struggle for existence, but on a struggle between the 
males for possession of the females ; the result is not 
death to the unsuccessful competitor, but few or no 
offspring. Sexual selection is, therefore, less rigorous 
than natural selection. Generally, the most vigorous 
males, those which are best fitted for their places in 
nature, will leave most progeny. But in many cases, 
victory will depend not on general vigour, but on having 
special weapons, confined to the male sex. A hornless 
stag or spurless cock would have a poor chance of 
leaving offspring. Sexual selection by always allowing 
the victor to breed might surely give indomitable 
courage, length to the spur, and strength to the wing 
to strike in the spurred leg, as well as the brutal cock- 
fighter, who knows well that he can improve his breed 
by careful selection of the best cocks. How low in 
the scale of nature this law of battle descends, I know 
not ; male alligators have been described as fighting, 
bellowing, and whirling round, like Indians in a war- 
dance, for the possession of the females ; male salmons 
have been seen fighting all day long ; male stag-beetles 
often bear wounds from the huge mandibles of other 
males. The war is, perhaps, severest between the males 
of polygamous animals, and these seem often est pro- 
vided with special weapons. The males of carnivorous 
animals are already well armed ; though to them and to 
others, special means of defence may be given through 
means of sexual selection, as the mane to the lion, the 
shoulder-pad to the boar, and the hooked jaw to the 
male salmon ; for the shield may be as important for 
victory, as the sword or spear. 

Amongst birds, the contest is often of a more peaceful 
character. All those who have attended to the subject, 


believe that there is the severest rivalry between the 
males of many species to attract by singing the females. 
The rock-thrush of Guiana, birds of Paradise, and some 
others, congregate; and successive males display their 
gorgeous plumage and perform strange antics before the 
females, which standing by as spectators, at last choose 
the most attractive partner. Those who have closely 
attended to birds in confinement well know that they 
often take individual preferences and dislikes : thus 
Sir E. Heron has described how one pied peacock was 
eminently attractive to all his hen birds. It may 
appear childish to attribute any effect to such appa- 
rently weak means : I cannot here enter on the details 
necessary to support this view ; but if man can in a short 
time give elegant carriage and beauty to his bantams, 
according to his standard of beauty, I can see no good 
reason to doubt that female birds, by selecting, during 
thousands of generations, the most melodious or beautiful 
males, according to their standard of beauty, might pro- 
duce a marked effect. I strongly suspect that some 
well-known laws with respect to the plumage of male 
and female birds, in comparison with the plumage of the 
young, can be explained on the view of plumage having 
been chiefly modified by sexual selection, acting when 
the birds have come to the breeding age or during the 
breeding season ; the modifications thus produced being 
inherited at corresponding ages or seasons, either by 
the males alone, or by the males and females ; but I 
have not space here to enter on this subject. 

Thus it is, as I believe, that when the males and 
females of any animal have the same general habits 
of life, but differ in structure, colour, or ornament, 
such differences have been mainly caused by sexual 
selection ; that is, individual males have had, in suc- 
cessive generations, some slight advantage over other 


males, in their weapons, means of defence, or charms ; 
and have transmitted these advantages to their male 
offspring. Yet, I would not wish to attribute all such 
sexual differences to this agency: for we see peculi- 
arities arising and becoming attached to the male sex 
in our domestic animals (as the wattle in male carriers, 
horn-like protuberances in the cocks of certain fowls, 
&c), which we cannot believe to be either useful to the 
males in battle, or attractive to the females. We see 
analogous cases under nature, for instance, the tuft of 
hair on the breast of the turkey-cock, which can hardly 
be either useful or ornamental to this bird; — indeed, 
had the tuft aj)peared under domestication, it would 
have been called a monstrosity. 

Illustrations of the action of Natural Selection. — In 
order to make it clear how, as I believe, natural selec- 
tion acts, I must beg permission to give one or two 
imaginary illustrations. Let us take the case of a wolf, 
which preys on various animals, securing some by craft, 
some by strength, and some by fleetness; and let us 
suppose that the fleetest prey, a deer for instance, had 
from any change in the country increased in numbers, 
or that other prey had decreased in numbers, during 
that season of the year when the wolf is hardest pressed 
for food. I can under such circumstances see no reason 
to doubt that the swiftest and slimmest wolves would 
have the best chance of surviving, and so be preserved 
or selected, — provided always that they retained strength 
to master their prey at this or at some other period of 
the year, when they might be compelled to prey on 
other animals. I can see no more reason to doubt this, 
than that man can improve the fleetness of Iris grey- 
hounds by careful and methodical selection, or by that 
unconscious selection which results from each man trying 


to keep the best dogs without any thought of modifying 
the breed. 

Even without any change in the proportional numbers 
of the animals on which our wolf preyed, a cub might 
be born with an innate tendency to pursue certain kinds 
of prey. Nor can this be thought very improbable ; for 
we often observe great differences in the natural ten- 
dencies of our domestic animals ; one cat, for instance, 
taking to catch rats, another mice ; one cat, according to 
Mr. St. John, bringing home winged game, another hares 
or rabbits, and another hunting on marshy ground and 
almost nightly catching woodcocks or snipes. The ten- 
dency to catch rats rather than mice is known to be in- 
herited. Now, if any slight innate change of habit or of 
structure benefited an individual wolf, it would have 
the best chance of surviving and of leaving offspring. 
Some of its young would probably inherit the same 
habits or structure, and by the repetition of this process, 
a new variety might be formed which would either sup- 
plant or coexist with the parent-form of wolf. Or, again, 
the wolves inhabiting a mountainous district, and those 
frequenting the lowlands, would naturally be forced to 
hunt different prey ; and from the continued preserva- 
tion of the individuals best fitted for the two sites, two 
varieties might slowly be formed. These varieties would 
cross and blend where they met ; but to this subject of 
intercrossing we shall soon have to return. I may add, 
that, according to Mr. Pierce, there are two varieties of 
the wolf inhabiting the Catskill Mountains in the United 
States, one with a light greyhound-like form, which pur- 
sues deer, and the other more bulky, with shorter legs, 
which more frequently attacks the shepherd's flocks. 

Let us now take a more complex case. Certain plants 
excrete a sweet juice, apparently for the sake of elimi- 
nating something injurious from their sap : this is 


effected by glands at the base of the stipules in some 
Leguminosse, and at the back of the leaf of the common 
laurel. This juice, though small in quantity, is greedily 
sought by insects. Let us now suppose a little sweet 
juice or nectar to be excreted by the inner bases of the 
petals of a flower. In this case insects in seeking the 
nectar would get dusted with pollen, and would certainly 
often transport the pollen from one flower to the stigma 
of another flower. The flowers of two distinct individuals 
of the same species would thus get crossed ; and the act 
of crossing, we have good reason to believe (as will 
hereafter be more fully alluded to), would produce very 
vigorous seedlings, which consequently would have the 
best chance of flourishing and surviving. Some of these 
seedlings would probably inherit the nectar-excreting 
power. Those individual flowers which had the largest 
glands or nectaries, and which excreted most nectar, 
would be oftenest visited by insects, and would be 
oftenest crossed ; and so in the long-run would gain 
the upper hand. Those flowers, also, which had their 
stamens and pistils placed, in relation to the size and 
habits of the particular insects which visited them, so as 
to favour in any degree the transportal of their pollen 
from flower to flower, would likewise be favoured or se- 
lected. We might have taken the case of insects 
visiting flowers for the sake of collecting pollen instead 
of nectar ; and as pollen is formed for the sole object 
of fertilisation, its destruction appears a simple loss to 
the plant ; yet if a little pollen were carried, at first 
occasionally and then habitually, by the pollen-devour- 
ing insects from flower to flower, and a cross thus 
effected, although nine-tenths of the pollen were de- 
stroyed, it might still be a great gain to the plant ; and 
those individuals which produced more and more pollen, 
and had larger and larger anthers, would be selected. 


When our plant, by this process of the continued 
preservation or natural selection of more and more 
attractive flowers, had been rendered highly attractive 
to insects, they would, unintentionally on their part, 
regularly carry pollen from flower to flower ; and that 
they can most effectually do this, I could easily show 
by many striking instances. I will give only one — not 
as a very striking case, but as likewise illustrating one 
step in the separation of the sexes of plants, presently 
to be alluded to. Some holly-trees bear only male 
flowers, which have four stamens producing rather a 
small quantity of pollen, and a rudimentary pistil ; 
other holly-trees bear only female flowers ; these have a 
full-sized pistil, and four stamens with shrivelled anthers', 
in which not a grain of pollen can be detected. Having 
found a female tree exactly sixty yards from a male 
tree, I put the stigmas of twenty flowers, taken from 
different branches, under the microscope, and on all, 
without exception, there were pollen-grains, and on 
some a profusion of pollen. As the wind had set for 
several days from the female to the male tree, the 
pollen could not thus have been carried. The weather 
had been cold and boisterous, and therefore not favour- 
able to bees, nevertheless every female flower which 
I examined had been effectually fertilised by the bees, 
accidentally dusted with pollen, having flown from tree 
to tree in search of nectar. But to return to our 
imaginary case : as soon as the plant had been ren- 
dered so highly attractive to insects that pollen was 
regularly carried from flower to flower, another 
process might commence. No naturalist doubts the 
advantage of what has been called the "physiological 
division of labour ; " hence we may believe that it would 
be advantageous to a plant to produce stamens alone in 
one flower or on one whole plant, and pistils alone in 


another flower or on another plant. In plants under 
culture and placed under new conditions of life, some- 
times the male organs and sometimes the female organs 
become more or less impotent ; now if we suppose this to 
occur in ever so slight a degree under nature, then as 
pollen is already carried regularly from flower to flower, 
and as a more complete separation of the sexes of our 
plant would be advantageous on the principle of the 
division of labour, individuals with this tendency more 
and more increased, would be continually favoured or 
selected, until at last a complete separation of the sexes 
would be effected. 

Let us now turn to the nectar-feeding insects in our 
imaginary case : we may suppose the plant of which 
we have been slowly increasing the nectar by continued 
selection, to be a common plant ; and that certain in- 
sects depended in main part on its nectar for food. I 
could give many facts, showing how anxious bees are 
to save time ; for instance, their habit of cutting 
holes and sucking the nectar at the bases of certain 
flowers, which they can, with a very little more trouble, 
enter by the mouth. Bearing such facts in mind, I can 
see no reason to doubt that an accidental deviation in 
the size and form of the body, or in the curvature and 
length of the proboscis, &c, far too slight to be appre- 
ciated by us, might profit a bee or other insect, so that 
an individual so characterised would be able to obtain 
its food more quickly, and so have a better chance of 
living and leaving descendants. Its descendants would 
probably inherit a tendency to a similar slight deviation 
of structure. The tubes of the corollas of the common 
red and incarnate clovers (Trifolium pratense and in- 
carnatum) do not on a hasty glance appear to differ in 
length ; yet the hive-bee can easily suck the nectar out 
of the incarnate clover, but not out of the common red 


clover, which is visited by humble-bees alone ; so that 
whole fields of the red clover offer in vain an abundant 
supply of precious nectar to the hive-bee. Thus it 
might be a great advantage to the hive-bee to have a 
slightly longer or differently constructed proboscis. On 
the other hand, I have found by experiment that the 
fertility of clover greatly depends on bees visiting and 
moving parts of the corolla, so as to push the pollen 
on to the stigmatic surface. Hence, again, if humble- 
bees were to become rare in any country, it might 
be a great advantage to the red clover to have a 
shorter or more deeply divided tube to its corolla, so 
that the hive-bee could visit its flowers. Thus I can 
understand how a flower and a bee might slowly be- 
come, either simultaneously or one after the other, 
modified and adapted in the most perfect manner to 
each other, by the continued preservation of individuals 
presenting mutual and slightly favourable deviations of 

I am well aware that this doctrine of natural selection, 
exemplified in the above imaginary instances, is open to 
the same objections which were at first urged against 
Sir Charles Lyell's noble views on "the modern changes 
of the earth, as illustrative of geology ;" but we now 
very seldom hear the action, for instance, of the coast- 
waves, called a trifling and insignificant cause, when 
applied to the excavation of gigantic valleys or to the 
formation of the longest lines of inland cliffs. Natural 
selection can act only by the preservation and accumula- 
tion of infinitesimally small inherited modifications, each 
profitable to the preserved being; and as modern 
geology has almost banished such views as the excava- 
tion of a great valley by a single diluvial wave, so 
will natural selection, if it be a true principle, banish 
the belief of the continued creation of new organic 


beings, or of any great and sudden modification in their 

On the Intercrossing of Individuals. — I must here 
introduce a short digression. In the case of animals 
and plants with separated sexes, it is of course obvious 
that two individuals must always unite for each birth ; 
but in the case of hermaphrodites this is far from ob- 
vious. Nevertheless I am strongly inclined to believe 
that with all hermaphrodites two individuals, either 
occasionally or habitually, concur for the reproduction of 
their kind. This view, I may add, was first suggested 
by Andrew Knight. We shall presently see its im- 
portance ; but I must here treat the subject with extreme 
brevity, though I have the materials prepared for an 
ample discussion. All vertebrate animals, all insects, 
and some other large groups of animals, pair for each 
birth. Modern research has much diminished the 
number of supposed hermaphrodites, and of real her- 
maphrodites a large number pair ; that is, two indivi- 
duals regularly unite for reproduction, which is all that 
concerns us. But still there are many hermaphrodite 
animals which certainly do not habitually pair, and a 
vast majority of plants are hermaphrodites. What 
reason, it may be asked, is there for supposing in these 
cases that two individuals ever concur in reproduction ? 
As it is impossible here to enter on details, I must trust 
to some general considerations alone. 

In the first place, I have collected so large a body of 
facts, showing, in accordance with the almost universal 
belief of breeders, that with animals and plants a cross 
between different varieties, or between individuals of 
the same variety but of another strain, gives vigour and 
fertility to the offspring ; and on the other hand, that 
close interbreeding diminishes vigour and fertility ; that 


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these facts alone incline me to believe that it is a 
general law of nature (utterly ignorant though we be 
of the meaning of the law) that no organic being self- 
fertilise^itselfjfor an eternity of generations ; but that a 
cross with another individual is occasionally — perhaps 
at very long intervals — indispensable. 

On the belief that this is a law of nature, we can, I 
think, understand several large classes of facts, such as 
the following, which on any other view are inexplicable. 
Every hybridizer knows how unfavourable exposure to 
wet is to the fertilisation of a flower, yet what a multi- 
tude of flowers have their anthers and stigmas fully 
exposed to the weather! but if an occasional cross be 
indispensable, the fullest freedom for the entrance of 
pollen from another individual will explain this state 
of exposure, more especially as the plant's own anthers 
and pistil generally stand so close together that self- 
fertilisation seems almost inevitable. Many flowers, on 
the other hand, have their organs of fructification closely 
enclosed, as in the great papilionaceous or pea-family ; 
but in several, perhaps in all, such flowers, there is a 
very curious adaptation between the structure of the 
flower and the manner in which bees suck the nectar ; 
for, in doing this, they either push the flower's own pollen 
on the stigma, or bring pollen from another flower. So 
necessary are the visits of bees to papilionaceous flowers, 
that I have found, by experiments published elsewhere, 
that their fertility is greatly diminished if these visits 
be prevented. Now, it is scarcely possible that bees 
should fly from flower to flower, and not carry pollen 
from one to the other, to the great good, as I believe, 
of the plant. Bees will act like a camel-hair pencil, 
and it is quite sufficient just to touch the anthers of 
one flower and then the stigma of another with the 
same brush to ensure fertilisation ; but it must not be 


supposed that bees would thus produce a multitude of 
hybrids between distinct species ; for if you bring on the 
same brush a plant's own pollen and pollen from another 
species, the former will have such a prepotent effect, 
that it will invariably and completely destroy, as has been 
shown by Gartner, any influence from the foreign pollen. 
When the stamens of a flower suddenly spring towards 
the pistil, or slowly move one after the other towards it, 
the contrivance seems adapted solely to ensure self- 
fertilisation ; and no doubt it is useful for this end : but, 
the agency of insects is often required to cause the 
stamens to spring forward, as Kolreuter has shown to 
be the case with the barberry ; and curiously in this 
very genus, which seems to have a special contrivance 
for self-fertilisation, it is well known that if very closely- 
allied forms or varieties are planted near each other, it 
is hardly possible to raise pure seedlings, so largely do 
they naturally cross. In many other cases, far from 
there being any aids for self-fertilisation, there are 
special contrivances, as I could show from the writings 
of C. C. Sprengel and from my own observations, which 
effectually prevent the stigma receiving pollen from its 
own flower : for instance, in Lobelia fulgens, there is a 
really beautiful and elaborate contrivance by which 
every one of the infinitely numerous pollen-granules 
are swept out of the conjoined anthers of each flower, 
before the stigma of that individual flower is ready to 
receive them ; and as this flower is never visited, at least 
in my garden, by insects, it never sets a seed, though by 
placing pollen from one flower on the stigma of another, 
I raised plenty of seedlings ; and whilst another species 
of Lobelia growing close by, which is visited by bees, 
seeds freely. In very many other cases, though there 
be no special mechanical contrivance to prevent the 
stigma of a flower receiving its own pollen, yet, as 


C. C. Sprengel has shown, and as I can confirm, either the 
anthers burst before the stigma is ready for fertilisation, 
or the stigma is ready before the pollen of that flower 
is ready, so that these plants have in fact separated 
sexes, and must habitually be crossed. How strange 
are these facts ! How strange that the pollen and stig- 
matic surface of the same flower, though placed so close 
together, as if for the very purpose of self-fertilisation, 
should in so many cases be mutually useless to each 
other! How simply are these facts explained on the 
view of an occasional cross with a distinct individual 
being advantageous or indispensable ! 

If several varieties of the cabbage, radish, onion, and 
of some other plants, be allowed to seed near each other, 
a large majority, as I have found, of the seedlings thus 
raised will turn out mongrels : for instance, I raised 233 
seedling cabbages from some plants of different varieties 
growing near each other, and of these only 78 were true 
to their kind, and some even of these were not perfectly 
true. Yet the pistil of each cabbage-flower is sur- 
rounded not only by its own six stamens, but by those 
of the many other flowers on the same plant. How, 
then, comes it that such a vast number of the seedlings 
are mongrelized ? I suspect that it must arise from the 
pollen of a distinct variety having a prepotent effect 
over a flower's own pollen ; and that this is part of 
the general law of good being derived from the inter- 
crossing of distinct individuals of the same species. 
When distinct species are crossed the case is directly 
the reverse, for a plant's own pollen is always prepotent 
over foreign pollen ; but to this subject we shall return 
in a future chapter. 

In the case of a gigantic tree covered with innume- 
rable flowers, it may be objected that pollen could seldom 
be carried from tree to tree, and at most only from flower 



to flower on the same tree, and that flowers on the same 
tree can be considered as distinct individuals only in a 
limited sense. I believe this objection to be valid, but 
that nature has largely provided against it by giving to 
trees a strong tendency to bear flowers with separated 
sexes. When the sexes are separated, although the 
male and female flowers may be produced on the same 
tree, we can see that pollen must be regularly carried 
from flower to flower ; and this will give a better chance 
of pollen being occasionally carried from tree to tree. 
That trees belonging to all Orders have their sexes 
more often separated than other plants, I find to be the 
case in this country; and at my request Dr. Hooker 
tabulated the trees of New Zealand, and Dr. Asa Gray 
those of the United States, and the result was as I anti- 
cipated. On the other hand, Dr. Hooker has recently 
informed me that he finds that the rule does not hold 
in Australia ; and I have made these few remarks on 
the sexes of trees simply to call attention to the subject. 
Turning for a very brief space to animals : on the 
land there are some hermaphrodites, as land-mollusca 
and earth-worms ; but these all pair. As yet I have not 
found a single case of a terrestrial animal which fer- 
tilises itself. We can understand this remarkable fact, 
which offers so strong a contrast with terrestrial plants, 
on the view of an occasional cross being indispensable, 
by considering the medium in which terrestrial animals 
live, and the nature of the fertilising element ; for we 
know of no means, analogous to the action of insects and 
of the wind in the case of plants, by which an occasional 
cross could be effected with terrestrial animals without 
the concurrence of two individuals. Of aquatic animals, 
there are many self-fertilising hermaphrodites ; but here 
currents in the water offer an obvious means for an occa- 
sional cross. And, as in the case of flowers, I have as yet 


failed, after consultation with one of the highest autho- 
rities, namely, Professor Huxley, to discover a single case 
of an hermaphrodite animal with the organs of reproduc- 
tion so perfectly enclosed within the body, that access 
from without and the occasional influence of a distinct 
individual can be shown to be physically impossible. 
Cirripedes long appeared to me to present a case of 
very great difficulty under this point of view ; but I 
have been enabled, by a fortunate chance, elsewhere to 
prove that two individuals, though both are self-fer- 
tilising hermaphrodites, do sometimes cross. 

It must have struck most naturalists as a strange 
anomaly that, in the case of both animals and plants, 
species of the same family and even of the same genus, 
though agreeing closely with each other in almost their 
whole organisation, yet are not rarely, some of them 
hermaphrodites, and some of them unisexual. But if, in 
fact, all hermaphrodites do occasionally intercross with 
other individuals, the difference between hemaphroclites 
and unisexual species, as far as function is concerned, 
becomes very small. 

From these several considerations and from the many 
special facts which I have collected, but which I am 
not here able to give, I am strongly inclined to suspect 
that, both in the vegetable and animal kingdoms, an 
occasional intercross with a distinct individual is a law 
of nature. I am well aware that there are, on this view, 
many cases of difficulty, some of which I am trying to 
investigate. Finally then, we may conclude that in 
many organic beings, a cross between two individuals is 
an obvious necessity for each birth ; in many others it 
occurs perhaps only at long intervals ; but in none, as I 
suspect, can self-fertilisation go on for perpetuity. 

Circumstances favourable to Natural Selection. — This 


is an extremely intricate subject. A large amount of 
inheritable and diversified variability is favourable, but 
I believe mere individual differences suffice for the work. 
A large number of individuals, by giving a better chance 
for the appearance within any given period of profitable 
variations, will compensate for a lesser amount of vari- 
ability in each individual, and is, I believe, an ex- 
tremely important element of success. Though nature 
grants vast periods of time for the work of natural 
selection, she does not grant an indefinite period ; for 
as all organic beings are striving, it may be said, to 
seize on each place in the economy of nature, if any one 
species does not become modified and improved in a 
corresponding degree with its competitors, it will soon 
be exterminated. 

In man's methodical selection, a breeder selects for 
some definite object, and free intercrossing will wholly 
stop his work. But when many men, without intending 
to alter the breed, have a nearly common standard of 
perfection, and all try to get and breed from the best 
animals, much improvement and modification surely but 
slowly follow from this unconscious process of selection, 
notwithstanding a large amount of crossing with inferior 
animals. Thus it will be in nature ; for within a con- 
fined area, with some place in its polity not so perfectly 
occupied as might be, natural selection will always tend 
to preserve all the individuals varying in the right direc- 
tion, though in different degrees, so as better to fill up 
the unoccupied place. But if the area be large, its 
several districts will almost certainly present different 
conditions of life ; and then if natural selection be mo- 
difying and improving a species in the several districts, 
there will be intercrossing with the other individuals 
of the same species on the confines of each. And in 
this case the effects of intercrossing can hardly be coun- 


terbalancecl by natural selection always tending to mo- 
dify all the individuals in each district in exactly the 
same manner to the conditions of each ; for in a con- 
tinuous area, the conditions will generally graduate 
away insensibly from one district to another. The in- 
tercrossing will most affect those animals which unite 
for each birth, which wander much, and which do not 
breed at a very quick rate. Hence in animals of this 
nature, for instance in birds, varieties will generally be 
confined to, .separated countries ; and this I believe to be 
the case. In hermaphrodite organisms which cross only 
occasionally, and likewise in animals which unite for 
each birth, but which wander little and which can in- 
crease at a very rapid rate, a new and improved variety 
might be quickly formed on any one spot, and might 
there maintain itself in a body, so that whatever inter- 
crossing took place would be chiefly between the indi- 
viduals of the same new variety. A local variety when 
once thus formed might subsequently slowly spread to 
other districts. On the above principle, nurserymen 
always prefer getting seed from a large body of plants 
of the same variety, as the chance of intercrossing with 
other varieties is thus lessened. 

Even in the case of slow-breeding animals, which 
unite for each birth, we must not overrate the effects 
of intercrosses in retarding natural selection ; for I can 
bring a considerable catalogue of facts, showing that 
within the same area, varieties of the same animal can 
long remain distinct, from haunting different stations, 
from breeding at slightly different seasons, or from 
varieties of the same kind preferring to pair together. 

Intercrossing plays a very important part in nature 
in keeping the individuals of the same species, or of the 
same variety, true and uniform in character. It will 
obviously thus act far more efficiently with those animals 


which unite for each birth ; but I have already attempted 
to show that we have reason to believe that occasional 
intercrosses take place with all animals and with all 
plants. Even if these take place only at long intervals, 
I am convinced that the young thus produced will gain 
so much in vigour and fertility over the offspring from 
long-continued self-fertilisation, that they will have a 
better chance of surviving and propagating their kind ; 
and thus, in the long run, the influence of intercrosses, 
even at rare intervals, will be great. If there exist 
organic beings which never intercross, uniformity of 
character can be retained amongst them, as long as 
their conditions of life remain the same, only through 
the principle of inheritance, and through natural selec- 
tion destroying any which depart from the proper type ; 
but if their conditions of life change and they undergo 
modification, uniformity of character can be given to 
their modified offspring, solely by natural selection pre- 
serving the same favourable variations. 

Isolation, also, is an important element in the process 
of natural selection. In a confined or isolated area, if 
not very large, the organic and inorganic conditions of 
life will generally be in a great degree uniform ; so that 
natural selection will tend to modify all the individuals 
of a varying species throughout the area in the same 
manner in relation to the same conditions. Intercrosses, 
also, with the individuals of the same species, which 
otherwise would have inhabited the surrounding and dif- 
ferently circumstanced districts, will be prevented. But 
isolation probably acts more efficiently in checking the 
immigration of better adapted organisms, after any phy- 
sical change, such as of climate or elevation of the land, 
&c. ; and thus new places in the natural economy of 
the country are left open for the old inhabitants to 
struggle for, and become adapted to, through modifica- 


tions in their structure and constitution. Lastly, isola- 
tion, by checking immigration and consequently com- 
petition, will give time for any new variety to be slowly 
improved ; and this may sometimes be of importance in 
the production of new species. If, however, an isolated 
area be very small, either from being surrounded by 
barriers, or from having very peculiar physical condi- 
tions, the total number of the individuals supported on 
it w T ill necessarily be very small ; and fewness of indivi- 
duals will greatly retard the production of new species 
through natural selection, by decreasing the chance of 
the appearance of favourable variations. 

If we turn to nature to test the truth of these re- 
marks, and look at any small isolated area, such as an 
oceanic island, although the total number of the species 
inhabiting it, will be found to be small, as we shall see 
in our chapter on geographical distribution; yet of 
these species a very large proportion are endemic, — that 
is, have been produced there, and nowhere else. Hence 
an oceanic island at first sight seems to have been 
highly favourable for the production of new species. 
But we may thus greatly deceive ourselves, for to ascer- 
tain whether a small isolated area, or a large open area 
like a continent, has been most favourable for the produc- 
tion of new organic forms, we ought to make the compari- 
son within equal times; and this we are incapable of doing. 

Although I do not doubt that isolation is of consider- 
able importance in the production of new species, on 
the whole I am inclined to believe that largeness of 
area is of more importance, more especially in the 
production of species, which will prove capable of 
enduring for a long period, and of spreading widely. 
Throughout a great and open area, not only will there 
be a better chance of favourable variations arising from 
the large number of individuals of the same species 



there supported, but the conditions of life are infinitely 
complex from the large number of already existing 
species; and if some of these many species become 
modified and improved, others will have to be improved 
in a corresponding degree or they will be exterminated. 
Each new form, also, as soon as it has been much im- 
proved, will be able to spread over the open and con- 
tinuous area, and will thus come into competition with 
many others. Hence more new places will be formed, 
and the competition to fill them will be more severe, on 
a large than on a small and isolated area. Moreover, 
great areas, though now continuous, owing to oscillations 
of level, will often have recently existed in a broken con- 
dition, so that the good effects of isolation will generally, 
to a certain extent, have concurred. Finally, I conclude 
that, although small isolated areas probably have been 
in some respects highly favourable for the production 
of new species, yet that the course of modification will 
generally have been more rapid on large areas; and 
what is more important, that the new forms produced 
on large areas, which already have been victorious over 
many competitors, will be those that will spread most 
widely, will give rise to most new varieties and species, 
and will thus play an important part in the changing 
history of the organic world. 

We can, perhaps, on these views, understand some 
facts which will be again alluded to in our chapter on 
geographical distribution ; for instance, that the pro- 
ductions of the smaller continent of Australia have 
formerly yielded, and apparently are now yielding, 
before those of the larger Europaeo- Asiatic area. Thus, 
also, it is that continental productions have everywhere 
become so largely naturalised on islands. On a small 
island, the race for life will have been less severe, and 
there will have been less modification and less exter- 


ruination. Hence, perhaps, it comes that the flora of 
Madeira, according to Oswald Heer, resembles the extinct 
tertiary flora of Europe. All fresh-water basins, taken 
together, make a small area compared with that of the 
sea or of the land ; and, consequently, the competition 
between fresh-water productions will have been less 
severe than elsewhere ; new forms will have been more 
slowly formed, and old forms more slowly exterminated. 
And it is in fresh water that we find seven genera of 
Ganoid fishes, remnants of a once preponderant order : 
and in fresh water we find some of the most anomalous 
forms now known in the world, as the Ornithorhynchus 
and Lepidosiren, which, like fossils, connect to a certain 
extent orders now widely separated in the natural scale. 
These anomalous forms may almost be called living 
fossils; they have endured to the present day, from 
having inhabited a confined area, and from having thus 
been exposed to less severe competition. 

To sum up the circumstances favourable and un- 
favourable to natural selection, as far as the extreme 
intricacy of the subject permits. I conclude, looking 
to the future, that for terrestrial productions a large 
continental area, which will probably undergo many 
oscillations of level, and which consequently will exist 
for long periods in a broken condition, will be the most 
favourable for the production of many new forms of life, 
likely to endure long and to spread widely. For the area 
will first have existed as a continent, and the inhabitants, 
at this period numerous in individuals and kinds, will 
have been subjected to very severe competition. When 
converted by subsidence into large separate islands, 
there will still exist many individuals of the same 
species on each island : intercrossing on the confines 
of the range of each species will thus be checked : after 
physical changes of any kind, immigration will be pre- 


vented, so that new places in the polity of each island 
will have to be filled up by modifications of the old in- 
habitants ; and time will be allowed for the varieties in 
each to become well modified and perfected. When, by 
renewed elevation, the islands shall be re-converted into 
a continental area, there will again be severe competi- 
tion : the most favoured or improved varieties will be 
enabled to spread: there will be much extinction of 
the less improved forms, and the relative proportional 
numbers of the various inhabitants of the renewed con- 
tinent will again be changed ; and again there will be 
a fair field for natural selection to improve still further 
the inhabitants, and thus produce new species. 

That natural selection will always act with extreme 
slowness, I fully admit. Its action depends on there 
being places in the polity of nature, which can be better 
occupied by some of the inhabitants of the country 
undergoing modification of some kind. The existence 
of such places will often depend on physical changes, 
which are generally very slow, and on the immigration 
of better adapted forms having been checked. But the 
action of natural selection will probably still oftener de- 
pend on some of the inhabitants becoming slowly modi- 
fied ; the mutual relations of many of the other inha- 
bitants being thus disturbed. Nothing can be effected, 
unless favourable variations occur, and variation itself is 
apparently always a very slow process. The process will 
often be greatly retarded by free intercrossing. Many 
will exclaim that these several causes are amply suffi- 
cient wholly to stop the action of natural selection. I do 
not believe so. On the other hand, I do believe that 
natural selection will always act very slowly, often only 
at long intervals of time, and generally on only a very 
few of the inhabitants of the same region at the same 
time. I further believe, that this very slow, intermit- 

Chap. IV. EXTINCTION". 109 

tent action of natural selection accords perfectly well 
with what geology tells us of the rate and manner at 
which the inhabitants of this world have changed. 

Slow though the process of selection may be, if feeble 
man can do much by his powers of artificial selection, 
I can see no limit to the amount of change, to the 
beauty and infinite complexity of the coadaptations 
between all organic beings, one with another and with 
their physical conditions of life, which may be effected in 
the long course of time by nature's power of selection. 

Extinction. — This subject will be more fully discussed 
in our chapter on Geology ; but it must be here alluded 
to from being intimately connected with natural selec- 
tion. Natural selection acts solely through the pre- 
servation of variations in some way advantageous, which 
consequently endure. But as from the high geometrical 
powers of increase of all organic beings, each area is 
already fully stocked with inhabitants, it follows that 
as each selected and favoured form increases in number, 
so will the less favoured forms decrease and become 
rare. Karity, as geology tells us, is the precursor to 
extinction. We can, also, see that any form repre- 
sented by few individuals will, during fluctuations 
in the seasons or in the number of its enemies, run 
a good chance of utter extinction. But we may go 
further than this ; for as new forms are continually 
and slowly being produced, unless we believe that the 
number of specific forms goes on perpetually and almost 
indefinitely increasing, numbers inevitably must be- 
come extinct. That the number of specific forms has 
not indefinitely increased, geology shows us plaiuly ; 
and indeed we can see reason why they should not 
have thus increased, for the number of places in the 
polity of nature is not indefinitely great, — not that we 


have any means of knowing that any one region has 
as yet got its maximum of species. Probably no region 
is as yet fully stocked, for at the Cape of Good Hope, 
where more species of plants are crowded together than 
in any other quarter of the world, some foreign plants 
have become naturalised, without causing, as far as we 
know, the extinction of any natives. 

Furthermore, the species which are most nume- 
rous in individuals will have the best chance of pro- 
ducing within any given period favourable variations. 
We have evidence of this, in the facts given in the 
second chapter, showing that it is the common species 
which afford the greatest number of recorded varieties, 
or incipient species. Hence, rare species will be less 
quickly modified or improved within any given period, 
and they will consequently be beaten in the race for life 
by the modified descendants of the commoner species. 

From these several considerations I think it in- 
evitably follows, that as new species in the course of 
time are formed through natural selection, others will 
become rarer and rarer, and finally extinct. The forms 
which stand in closest competition with those under- 
going modification and improvement, will naturally 
suffer most. And we have seen in the chapter on the 
Struggle for Existence that it is the most closely-allied 
forms, — varieties of the same species, and species of 
the same genus or of related genera, — which, from 
having nearly the same structure, constitution, and 
habits, generally come into the severest competition 
with each other. Consequently, each new variety or 
species, during the progress of its formation, will gene- 
rally press hardest on its nearest kindred, and tend to 
exterminate them. We see the same process of exter- 
mination amongst our domesticated productions, through 
the selection of improved forms by man. Many curious 


instances could be given showing how quickly new breeds 
of cattle, sheep, and other animals, and varieties of 
flowers, take the place of older and inferior kinds. In 
Yorkshire, it is historically known that the ancient black 
cattle were displaced by the long-horns, and that these 
" were swept away by the short-horns " (I quote the 
words of an agricultural writer) " as if by some mur- 
derous pestilence." 

Divergence of Character. — The principle, which I have 
designated by this term, is of high importance on my 
theory, and explains, as I believe, several important 
facts. In the first place, varieties, even strongly- 
marked ones, though having somewhat of the character 
of species — as is shown by the hopeless doubts in many 
cases how to rank them — yet certainly differ from each 
other far less than do good and distinct species. Never- 
theless, according to my view, varieties are species in 
the process of formation, or are, as I have called them, 
incipient species. How, then, does the lesser difference 
between varieties become augmented into the greater 
difference between species ? That this does habitually 
happen, we must infer from most of the innumerable 
species throughout nature presenting well-marked dif- 
ferences; whereas varieties, the supposed prototypes 
and parents of future well-marked species, present slight 
and ill-defined differences. Mere chance, as we may 
call it, might cause one variety to differ in some cha- 
racter from its parents, and the offspring of this variety 
again to differ from its parent in the very same cha- 
racter and in a greater degree ; but this alone would 
never account for so habitual and large an amount of 
difference as that between varieties of the same species 
and species of the same genus. 

As has always been my practice, let us seek light on 


this head from our domestic productions. We shall here 
find something analogous. A fancier is struck by a 
pigeon having a slightly shorter beak ; another fancier 
is struck by a pigeon having a rather longer beak ; and 
on the acknowledged principle that " fanciers do not 
and will not admire a medium standard, but like ex- 
tremes," they both go on (as has actually occurred 
with tumbler-pigeons) choosing and breeding from birds 
with longer and longer beaks, or with shorter and 
shorter beaks. Again, we may suppose that at an early 
period one man preferred swifter horses ; another 
stronger and more bulky horses. The early differences 
would be very slight ; in the course of time, from the 
continued selection of swifter horses by some breeders, 
and of stronger ones by others, the differences would 
become greater, and would be noted as forming two 
sub-breeds ; finally, after the lapse of centuries, the sub- 
breeds would become converted into two well-established 
and distinct breeds. As the differences slowly become 
greater, the inferior animals with intermediate cha- 
racters, being neither very swift nor very strong, will 
have been neglected, and will have tended to disappear. 
Here, then, we see in man's productions the action of 
what may be called the principle of divergence, causing 
differences, at first barely appreciable, steadily to in- 
crease, and the breeds to diverge in character both 
from each other and from their common parent. 

But how, it may be asked, can any analogous prin- 
ciple apply in nature ? I believe it can and does apply 
most efficiently, from the simple circumstance that the 
more diversified the descendants from any one species 
become in structure, constitution, and habits, by so much 
will they be better enabled to seize on many and widely 
diversified places in the polity of nature, and so be 
enabled to increase in numbers. 


We can clearly see this in the case of animals with 
simple habits. Take the case of a carnivorous quadru- 
ped, of winch the number that can be supported in any 
country has long ago arrived at its full average. If its 
natural powers of increase be allowed to act, it can 
succeed in increasing (the country not undergoing any 
change in its conditions) only by its varying descendants 
seizing on places at present occupied by other animals : 
some of them, for instance, being enabled to feed on 
new kinds of prey, either dead or alive ; some inhabiting 
new stations, climbing trees, frequenting water, and some 
perhaps becoming less carnivorous. The more diversi- 
fied in habits and structure the descendants of our car- 
nivorous animal became, the more places they would be 
enabled to occupy. What applies to one animal will 
apply throughout all time to all animals — that is, if they 
vary — for otherwise natural selection can do nothing. 
So it will be with plants. It has been experimentally 
proved, that if a plot of ground be sown with one species 
of grass, and a similar plot be sown with several distinct 
genera of grasses, a greater number of plants and a 
greater weight of dry herbage can thus be raised. The 
same has been found to hold good when first one variety 
and then several mixed varieties of wheat have been 
sown on equal spaces of ground. Hence, if any one 
species of grass were to go on varying, and those varie- 
ties were continually selected which differed from each 
other in at all the same manner as distinct species and 
genera of grasses differ from each other, a greater 
number of individual plants of this species of grass, in- 
cluding its modified descendants, would succeed in living 
on the same piece of ground. And we well know that 
each species and each variety of grass is annually 
sowing almost countless seeds ; and thus, as it may be 
said, is striving its utmost to increase its numbers. Con- 


sequently, I cannot doubt that in the course of many 
thousands of generations, the most distinct varieties of 
any one species of grass would always have the best 
chance of succeeding and of increasing in numbers, and 
thus of supplanting the less distinct varieties; and 
varieties, when rendered very distinct from each other, 
take the rank of species. 

The truth of the principle, that the greatest amount 
of life can be supported by great diversification of 
structure, is seen under many natural circumstances. 
In an extremely small area, especially if freely open to 
immigration, and where the contest between individual 
and individual must be severe, we always find great 
diversity in its inhabitants. For instance, I found that 
a piece of turf, three feet by four in size, which had been 
exposed for many years to exactly the same conditions, 
supported twenty species of plants, and these belonged 
to eighteen genera and to eight orders, which shows how 
much these plants differed from each other. So it is with 
the plants and insects on small and uniform islets ; and 
so in small ponds of fresh water. Farmers find that they 
can raise most food by a rotation of plants belonging to 
the most different orders : nature follows what may be 
called a simultaneous rotation. Most of the animals and 
plants which live close round any small piece of ground, 
could live on it (supposing it not to be in any way peculiar 
in its nature), and may be said to be striving to the utmost 
to live there ; but, it is seen, that where they come into the 
closest competition with each other, the advantages of 
diversification of structure, with the accompanying dif- 
ferences of habit and constitution, determine that the 
inhabitants, which thus jostle each other most closely, 
shall, as a general rule, belong to what we call different 
genera and orders. 
The same principle is seen in the naturalisation of 


plants through man's agency in foreign lands. It 
might have been expected that the plants which have 
succeeded in becoming naturalised in any land would 
generally have been closely allied to the indigenes ; for 
these are commonly looked at as specially created and 
adapted for their own country. It might, also, perhaps 
have been expected that naturalised plants would have 
belonged to a few groups more especially adapted to 
certain stations in their new homes. But the case is 
very different ; and Alph. De Candolle has well remarked 
in his great and admirable work, that floras gain by 
naturalisation, proportionally with the number of the 
native genera and species, far more in new genera than 
in new species. To give a single instance : in the last 
edition of Dr. Asa Gray's ' Manual of the Flora of the 
Northern United States,' 260 naturalised plants are 
enumerated, and these belong to 162 genera. We thus 
see that these naturalised plants are of a highly diversified 
nature. They differ, moreover, to a large extent from 
the indigenes, for out of the 162 genera, no less than 100 
genera are not there indigenous, and thus a large pro- 
portional addition is made to the genera of these States. 

By considering the nature of the plants or animals 
which have struggled successfully with the indigenes of 
any country, and have there become naturalised, we 
can gain some crude idea in what manner some of the 
natives would have had to be modified, in order to have 
gained an advantage over the other natives ; and we 
may, I think, at least safely infer that diversification of 
structure, amounting to new generic differences, would 
have been profitable to them. 

The advantage of diversification in the inhabitants of 
the same region is, in fact, the same as that of the 
physiological division of labour in the organs of the 
same individual body — a subject so well elucidated by 


Milne Edwards. No physiologist doubts that a stomach 
by being adapted to digest vegetable matter alone, or 
flesh alone, draws most nutriment from these substances. 
So in the general economy of any land, the more widely 
and perfectly the animals and plants are diversified for 
different habits of life, so will a greater number of 
individuals be capable of there supporting themselves. 
A set of animals, with their organisation but little diver- 
sified, could hardly compete with a set more perfectly 
diversified in structure. It may be doubted, for instance, 
whether the Australian marsupials, which are divided 
into groups differing but little from each other, and 
feebly representing, as Mr. Waterhouse and others have 
remarked, our carnivorous, ruminant, and rodent mam- 
mals, could successfully compete with these well-pro- 
nounced orders. In the Australian mammals, we see 
the process of diversification in an early and incomplete 
stage of development. 

After the foregoing discussion, which ought to have 
been much amplified, we may, I think, assume that the 
modified descendants of any one species will succeed by 
so much the better as they become more diversified in 
structure, and are thus enabled to encroach on places 
occupied by other beings. Now let us see how this 
principle of great benefit being derived from divergence 
of character, combined with the principles of natural 
selection and of extinction, will tend to act. 

The accompanying diagram will aid us in understand- 
ing this rather perplexing subject. Let A to L repre- 
sent the species of a genus large in its own country ; 
these species are supposed to resemble each other in 
unequal degrees, as is so generally the case in nature, 
and as is represented in the diagram by the letters 
standing at unequal distances. I have said a large 
genus, because we have seen in the second chapter, 

a 1 * 

'f 14 



f 14. 

O 24 " 


. m? 1 ' 

s 1 * 


r 1 

m 2 * 


V 1 

t 7 M 





! XIU 



\ i \ 




i ' / 







or 1 






x M 




''•■:/ 3 


W Q:.\/ 

...X. 9 


a 8 ■ .,: 



l< "■ : >, 

im 8 

u 8 

w s '<■.;.■'' 




f ? 

... k 

■■Aii 7 '•• 

Vm, 7 


1//' •■'■::■; 

\ ■ :/ 7 



r 6 

'ik t: 


i **>i 

//~ fi 



cl 5 


' : $.-"m- 

<v ' 

..-•■' .5 


a? '•'■ 



''''<■: \/ 4. 

'■'■:■':. y/y * 



'' : H 3 

■Urn- 3 

S 3 

. % 5 



S 2 %7 

z : 

; ; e*\ 

• •'•'i a 


of' ■ mS 

•.'.■' z J 


A B C D E F 

G H I K L 

"WTWest ]ibh..Ha.-tton.GacclHn.. 


that on an average more of the species of large genera 
vary than of small genera; and the varying species of 
the large genera present a greater number of varieties. 
We have, also, seen that the species, which are the 
commonest • and the most widely-diffused, vary more 
than rare species with restricted ranges. Let (A) be a 
common, widely-diffused, and varying species, belong- 
ing to a genus large in its own country. The little fan 
of diverging dotted lines of unequal lengths proceeding 
from (A), may represent its varying offspring. The 
variations are supposed to be extremely slight; but of 
the most diversified nature ; they are not supposed all 
to appear simultaneously, but often after long intervals 
of time ; nor are they all supposed to endure for equal 
periods. Only those variations which are in some way 
profitable will be preserved or naturally selected. And 
here the importance of the principle of benefit being- 
derived from divergence of character comes in ; for this 
will generally lead to the most different or divergent 
variations (represented by the outer dotted lines) being 
preserved and accumulated by natural selection. When 
a dotted line reaches one of the horizontal lines, and is 
there marked by a small numbered letter, a sufficient 
amount of variation is supposed to have been accu- 
mulated to have formed a fairly well-marked variety, 
such as would be thought worthy of record in a sys- 
tematic work. 

The intervals between the horizontal lines in the 
diagram, may represent each a thousand generations ; 
but it would have been better if each had represented 
ten thousand generations. After a thousand genera- 
tions, species (A) is supposed to have produced two 
fairly well-marked varieties, namely a} and m\ These 
two varieties will generally continue to be exposed to 
the same conditions which made their parents variable, 


and the tendency to variability is in itself hereditary, 
consequently they will tend to vary, and generally 
to vary in nearly the same manner as their parents 
varied. Moreover, these two varieties, being only 
slightly modified forms, will tend to inherit those ad- 
vantages which made ^their common parent (A) more 
numerous than most of the other inhabitants of the 
same country ; they will likewise partake of those more 
general advantages which made the genus to which the 
parent-species belonged, a large genus in its own country. 
And these circumstances we know to be favourable to 
the production of new varieties. 

If, then, these two varieties be variable, the most 
divergent of their variations will generally be preserved 
during the next thousand generations. And after this 
interval, variety a 1 is supposed in the diagram to have 
produced variety a 2 , which will, owing to the principle 
of divergence, differ more from (A) than did variety 
a\ Variety m l is supposed to have produced two 
varieties, namely m 2 and s 2 , differing from each other, 
and more considerably from their common parent (A). 
We may continue the process by similar steps for any 
length of time ; some of the varieties, after each thousand 
generations, producing only a single variety, but in a 
more and more modified condition, some producing two 
or three varieties, and some failing to produce any. 
Thus the varieties or modified descendants, proceeding 
from the common parent (A), will generally go on in- 
creasing in number and diverging in character. In 
the diagram the process is represented up to the ten- 
thousandth generation, and under a condensed and sim- 
plified form up to the fourteen-thousandth generation. 

But I must here remark that I do not suppose that 
the process ever goes on so regularly as is represented in 
the diagram, though in itself made somewhat irregular. 


I am for from thinking that the most divergent varieties 
will invariably prevail and multiply: a medium form 
may often long endure, and may or may not produce 
more than one modified descendant ; for natural selec- 
tion will always act according to the nature of the 
places which are either unoccupied or not perfectly 
occupied by other beings ; and this will depend on in- 
finitely complex relations. But as a general rule, the 
more diversified in structure the descendants from any 
one species can be rendered, the more places they will 
be enabled to seize on, and the more their modified 
progeny will be increased. In our diagram the line of 
succession is broken at regular intervals by small num- 
bered letters marking the successive forms which have 
become sufficiently distinct to be recorded as varieties. 
But these breaks are imaginary, and might have been 
inserted anywhere, after intervals long enough to have 
allowed the accumulation of a considerable amount of 
divergent variation. 

As all the modified descendants from a common and 
widely-diffused species, belonging to a large genus, will 
tend to partake of the same advantages which made 
their parent successful in life, they will generally go 
on multiplying in number as well as diverging in 
character: this is represented in the diagram by the 
several divergent branches proceeding from (A). The 
modified offspring from the later and more highly im- 
proved branches in the lines of descent, will, it is pro- 
bable, often take the place of, and so destroy, the 
earlier and less improved branches : this is represented 
in the diagram by some of the lower branches not reach- 
ing to the upper horizontal lines. In some cases I do 
not doubt that the process of modification will be con- 
fined to a single line of descent, and the number of the 
descendants will not be increased ; although the amount 


of divergent modification may have been increased in the 
successive generations. This case would be represented 
in the diagram, if all the lines proceeding from (A) 
were removed, excepting that from a 1 to a 10 . In the 
same way, for instance, the English race-horse and 
English pointer have apparently both gone on slowly 
diverging in character from their original stocks, with- 
out either having given off any fresh branches or races. 

After ten thousand generations, species (A) is supposed 
to have produced three forms, a 10 , / 10 , and m 10 , which, 
from having diverged in character during the successive 
generations, will have come to differ largely, but perhaps 
unequally, from each other and from their common 
parent. If we suppose the amount of change between 
each horizontal line in our diagram to be excessively 
small, these three forms may still be only well-marked 
varieties ; or they may have arrived at the doubtful 
category of sub-species ; but we have only to suppose 
the steps in the process of modification to be more 
numerous or greater in amount, to convert these three 
forms into well-defined species : thus the diagram 
illustrates the steps by which the small differences 
distinguishing varieties are increased into the larger 
differences distinguishing species. By continuing the 
same process for a greater number of generations (as 
shown in the diagram in a condensed and simplified 
manner), we get eight species, marked by the letters 
between a 14 and m 14 , all descended from (A). Thus, as I 
believe, species are multiplied and genera are formed. 

In a large genus it is probable that more than one 
species would vary. In the diagram I have assumed 
that a second species (I) has produced, by analogous 
steps, after ten thousand generations, either two well- 
marked varieties (tv 10 and z 10 ) or two species, according 
to the amount of change supposed to be represented be- 


tween the horizontal lines. After fourteen thousand 
generations, six new species, marked by the letters n li 
to z 1 4 , are supposed to have been produced. In each 
genus, the species, which are already extremely dif- 
ferent in character, will generally tend to produce the 
greatest number of modified descendants ; for these will 
have the best chance of filling new and widely different 
places in the polity of nature : hence in the diagram I 
have chosen the extreme species (A), and the nearly 
extreme species (I), as those which have largely varied, 
and have given rise to new varieties and species. The 
other nine species (marked by capital letters) of our 
original genus, may for a long period continue trans- 
mitting unaltered descendants; and this is shown in 
the diagram by the dotted lines not prolonged far up- 
wards from want of space. 

But during the process of modification, represented 
in the diagram, another of our principles, namely that 
of extinction, will have played an important part. As in 
each fully stocked country natural selection necessarily 
acts by the selected form having some advantage in the 
struggle for life over other forms, there will be a con- 
stant tendency in the improved descendants of any one 
species to supplant and exterminate in each stage of 
descent their predecessors and their original parent. 
For it should be remembered that the competition will 
generally be most severe between those forms which 
are most nearly related to each other in habits, constitu- 
tion, and structure. Hence all the intermediate forms 
between the earlier and later states, that is between the 
less and more improved state of a species, as well as 
the original parent-species itself, will generally tend to 
become extinct. So it probably will be with many whole 
collateral lines of descent, which will be conquered by 
later and improved lines of descent. If, however, the 



modified offspring of a species get into some distinct 
country, or become quickly adapted to some quite new 
station, in which child and parent do not come into 
competition, both may continue to exist. 

If then our diagram be assumed to represent a 
considerable amount of modification, species (A) and 
all the earlier varieties will have become extinct, 
having been replaced by eight new species (a 14 to m l4 ) ; 
and (I) will have been replaced by six (n u to z u ) new 

But we may go further than this. The original species 
of our genus were supposed to resemble each other in 
unequal degrees, as is so generally the case in nature ; 
species (A) being more nearly related to B, C, and D, 
than to the other species ; and species (I) more to G, H, 
K, L, than to the others. These two species (A) and (I), 
were also supposed to be very common and widely dif- 
fused species, so that they must originally have had 
some advantage over most of the other species of the 
genus. Their modified descendants, fourteen in number 
at the fourteen-thousandth generation, will probably 
have inherited some of the same advantages: they 
have also been modified and improved in a diversified 
manner at each stage of descent, so as to have become 
adapted to many related places in the natural economy 
of their country. It seems, therefore, to me extremely 
probable that they will have taken the places of, and 
thus exterminated, not only their parents (A) and (I), 
but likewise some of the original species which were 
most nearly related to their parents. Hence very few of 
the original species will have transmitted offspring to the 
fourteen-thousandth generation. We may suppose that 
only one (F), of the two species which were least closely 
related to the other nine original species, has transmitted 
descendants to this late stage of descent. 


The new species in our diagram descended from the 
original eleven species, will now be fifteen in number. 
Owing to the divergent tendency of natural selection, 
the extreme amount of difference in character between 
species a 14 and z w will be much greater than that 
between the most different of the original eleven species. 
The new species, moreover, will be allied to each other 
in a widely different manner. Of the eight descendants 
from (A) the three marked a u , q 14 , p 14 , will be nearly 
related from having recently branched off from a 10 ; b 14 
and / 14 , from having diverged at an earlier period from 
a 5 , will be in some degree distinct from the three first- 
named species ; and lastly, o 14 , e 14 , and m u , will be 
nearly related one to the other, but from having di- 
verged at the first commencement of the process of 
modification, will be widely different from the other 
Rye species, and may constitute a sub-genus or even a 
distinct genus. 

The six descendants from (I) will form two sub- 
genera or even genera. But as the original species (I) 
differed largely from (A), standing nearly at the extreme 
points of the original genus, the six descendants from 
(I) will, owing to inheritance, differ considerably from 
the eight descendants from (A) ; the two groups, more- 
over, are supposed to have gone on diverging in dif- 
ferent directions. The intermediate species, also (and 
this is a very important consideration), which connected 
the original species (A) and (I), have all become, ex- 
cepting (F), extinct, and have left no descendants. 
Hence the six new species descended from (I), and the 
eight descended from , (A), will have to be ranked as 
very distinct genera, or even as distinct sub-families. 

Thus it is, as I believe, that two or more genera 
are produced by descent, with modification, from two 
or more species of the same genus. And the two or 



more parent-species are supposed to have descended 
from some one species of an earlier genus. In our 
diagram, this is indicated by the broken lines, beneath 
the capital letters, converging in sub-branches down- 
wards towards a single point; this point representing 
a single species, the supposed single parent of our 
several new sub-genera and genera. 

It is worth while to reflect for a moment on the cha- 
racter of the new species f u , which is supposed not to 
have diverged much in character, but to have retained 
the form of (F), either unaltered or altered only in a 
slight degree. In this case, its affinities to the other 
fourteen new species will be of a curious and circuitous 
nature. Having descended from a form which stood 
between the two parent-species (A) and (I), now sup- 
posed to be extinct and unknown, it will be in some 
degree intermediate in character between the two 
groups descended from these species. But as these two 
groups have gone on diverging in character from the 
type of their parents, the new species (f u ) will not be 
directly intermediate between them, but rather between 
types of the two groups ; and every naturalist will be 
able to bring some such case before his mind. 

In the diagram, each horizontal line has hitherto been 
supposed to represent a thousand generations, but each 
may represent a million or hundred million generations, 
and likewise a section of the successive strata of the 
earth's crust including extinct remains. We shall, when 
we come to our chapter on Geology, have to refer again 
to this subject, and I think we shall then see that the 
diagram throws light on the affinities of extinct beings, 
which, though generally belonging to the same orders, 
or families, or genera, with those now living, yet are 
often, in some degree, intermediate in character between 
existing groups ; and we can understand this fact, for 


the extinct species lived at very ancient epochs when 
the branching lines of descent had diverged less. 

I see no reason to limit the process of modification, as 
now explained, to the formation of genera alone. If, in 
our diagram, we suppose the amount of change repre- 
sented by each successive group of diverging dotted lines 
to be very great, the forms marked a 14 to p 14 , those 
marked b ,4 and/ 14 , and those marked o 14 to m 14 , will 
form three very distinct genera. We shall also have two 
very distinct genera descended from (I) ; and as these 
latter two genera, both from continued divergence of 
character and from inheritance from a different parent, 
will differ widely from the three genera descended from 
(A), the two little groups of genera will form two distinct 
families, or even orders, according to the amount of 
divergent modification supposed to be represented in the 
diagram. And the two new families, or orders, will have 
descended from two species of the original genus ; and 
these two species are supposed to have descended from 
one species of a still more ancient and unknown genus. 

We have seen that in each country it is the species 
of the larger genera which oftenest present varieties or 
incipient species. This, indeed, might have been ex- 
pected ; for as natural selection acts through one form 
having some advantage over other forms in the struggle 
for existence, it will chiefly act on those which already 
have some advantage ; and the largeness of any group 
shows that its species have inherited from a common 
ancestor some advantage in common. Hence, the 
struggle for the production of new and modified de- 
scendants, will mainly lie between the larger groups, 
which are all trying to increase in number. One 
large group will slowly conquer another large group, 
reduce its numbers, and thus lessen its chance of further 
variation and improvement. Within the same large 


group, the later and more highly perfected sub-groups, 
from branching out and seizing on many new places 
in the polity of Nature, will constantly tend to supplant 
and destroy the earlier and less improved sub-groups. 
Small and broken groups and sub-groups will finally 
tend to disappear. Looking to the future, we can pre- 
dict that the groups of organic beings which are now 
large and triumphant, and which are least broken up, 
that is, which as yet have suffered least extinction, 
will for a long period continue to increase. But which 
groups will ultimately prevail, no man can predict; 
for we well know that many groups, formerly most 
extensively developed, have now become extinct. Look- 
ing still more remotely to the future, we may predict 
that, owing to the continued and steady increase of the 
larger groups, a multitude of smaller groups will become 
utterly extinct, and leave no modified descendants ; and 
consequently that of the species living at any one period, 
extremely few will transmit descendants to a remote 
futurity. I shall have to return to this subject in the 
chapter on Classification, but I may add that on this 
view of extremely few of the more ancient species 
having transmitted descendants, and on the view of all 
the descendants of the same species making a class, 
we can understand how it is that there exist but very 
few classes in each main division of the animal and 
vegetable kingdoms. Although extremely few of the 
most ancient species may now have living and modified 
descendants, yet at the most remote geological period, 
the earth may have been as well peopled with many 
species of many genera, families, orders, and classes, as 
at the present day. 

Summary of Chapter. — If during the long course of 
ages and under varying conditions of life, organic beings 

Chap. IV. SUMMARY. 127 

vary at all in the several parts of their organisation, and 
I think this cannot be disputed ; if there be, owing to the 
high geometrical powers of increase of each species, at 
some age, season, or year, a severe struggle for life, and 
this certainly cannot be disputed ; then, considering the 
infinite complexity of the relations of all organic beings 
to each other and to their conditions of existence, caus- 
ing an infinite diversity in structure, constitution, and 
habits, to be advantageous to them, I think it would 
be a most extraordinary fact if no variation ever had 
occurred useful to each being's own welfare, in the same 
way as so many variations have occurred useful to man. 
But if variations useful to any organic being do occur, 
assuredly individuals thus characterised will have the 
best chance of being preserved in the struggle for life ; 
and from the strong principle of inheritance they will 
tend to produce offspring similarly characterised. This 
principle of preservation, I have called, for the sake 
of brevity, Natural Selection. Natural selection, on the 
principle of qualities being inherited at corresponding 
ages, can modify the egg y seed, or young, as easily as 
the adult. Amongst many animals, sexual selection 
will give its aid to ordinary selection, by assuring to the 
most vigorous and best adapted males the greatest 
number of offspring. Sexual selection will also give 
characters useful to the males alone, in their struggles 
with other males. 

Whether natural selection has really thus acted in 
nature, in modifying and adapting the various forms of 
life to their several conditions and stations, must be 
judged of by the general tenour and balance of evidence 
given in the following chapters. But we already see 
how it entails extinction ; and how largely extinction 
has acted in the world's history, geology plainly de- 
clares. Natural selection, also, leads to divergence of 


character ; for more living beings can be supported on 
the same area the more they diverge in structure, habits, 
and constitution, of which we see proof by looking at 
the inhabitants of any small spot or at naturalised pro- 
ductions. Therefore during the modification of the 
descendants of any one species, and during the incessant 
struggle of all species to increase in numbers, the more 
diversified these descendants become, the better will be 
their chance of succeeding in the battle of life. Thus 
the small differences distinguishing varieties of the same 
species, will steadily tend to increase till they come to 
equal the greater differences between species of the 
same genus, or even of distinct genera. 

We have seen that it is the common, the widely- 
diffused, and widely-ranging species, belonging to the 
larger genera, which vary most ; and these will tend to 
transmit to their modified offspring that superiority 
which now makes them dominant in their own coun- 
tries. Natural selection, as has just been remarked, 
leads to divergence of character and to much extinction 
of the less improved and intermediate forms of life. On 
these principles, I believe, the nature of the affinities 
of all organic beings may be explained. It is a truly 
wonderful fact — the wonder of which we are apt to 
overlook from familiarity — that all animals and all 
plants throughout all time and space should be related 
to each other in group subordinate to group, in the 
manner which we everywhere behold — namely, vari- 
eties of the same species most closely related together, 
species of the same genus less closely and unequally 
related together, forming sections and sub-genera, spe- 
cies of distinct genera much less closely related, and 
genera related in different degrees, forming sub-fami- 
lies, families, orders, sub-classes, and classes. The 
several subordinate groups in any class cannot be 

Chap. IV. SUMMARY. 129 

ranked in a single file, but seem rather to be clustered 
round points, and these round other points, and so on 
in almost endless cycles. On the view that each spe- 
cies has been independently created, I can see no 
explanation of this great fact in the classification of all 
organic beings ; but, to the best of my judgment, it is 
explained through inheritance and the complex action 
of natural selection, entailing extinction and divergence 
of character, as we have seen illustrated in the diagram. 
The affinities of all the beings of the same class have 
sometimes been represented by a great tree. I believe 
this simile largely speaks the truth. The green and 
budding twigs may represent existing species ; and those 
produced during each former year may represent the 
long succession of extinct species. At each period of 
growth all the growing twigs have tried to branch out 
on all sides, and to overtop and kill the surrounding 
twigs and branches, in the same manner as species and 
groups of species have tried to overmaster other species 
in the great battle for life. The limbs divided into 
great branches, and these into lesser and lesser branches, 
were themselves once, when the tree was small, budding 
twigs ; and this connexion of the former and present 
buds by ramifying branches may well represent the 
classification of all extinct and living species in groups 
subordinate to groups. Of the many twigs which flou- 
rished when the tree was a mere bush, only two or 
three, now grown into great branches, yet survive and 
bear all the other branches ; so with the species which 
lived during long-past geological periods, very few now 
have living and modified descendants. From the first 
growth of the tree, many a limb and branch has decayed 
and dropped off; and these lost branches of various 
sizes may represent those whole orders, families, and 
genera which have now no living representatives, and 



which are known to us only from having been found in 
a fossil state. As we here and there see a thin strag- 
gling branch springing from a fork low down in a tree, 
and which by some chance has been favoured and is 
still alive on its summit, so we occasionally see an 
animal like the Ornithorhynchus or Lepidosiren, which 
in some small degree connects by its affinities two large 
branches of life, and which has apparently been saved 
from fatal competition by having inhabited a protected 
station. As buds give rise by growth to fresh buds, and 
these, if vigorous, branch out and overtop on all sides 
many a feebler branch, so by generation I believe it 
has been with the great Tree of Life, which fills with 
its dead and broken branches the crust of the earth, 
and covers the surface with its ever branching and 
beautiful ramifications. 



Laws of Variation. 

Effects of external conditions — Use and disuse, combined with 
natural selection ; organs of flight and of vision — Acclimatisa- 
tion — Correlation of growth — Compensation and economy of 
growth — False correlations — Multiple, rudimentary, and lowly 
organised structures variable — Parts developed in an unusual 
manner are highly variable : specific characters more variable 
than generic : secondary sexual characters variable — Species of 
the same genus vary in an analogous manner — Reversions to 
long lost characters — Summary. 

I have hitherto sometimes spoken as if the variations 
— so common and mnltiform in organic beings under 
domestication, and in a lesser degree in those in a state 
of nature — had been due to chance. This, of course, is 
a wholly incorrect expression, but it serves to acknow- 
ledge plainly our ignorance of the cause of each parti- 
cular variation. Some authors believe it to be as much 
the function of the reproductive system to produce 
individual differences, or very slight deviations of struc- 
ture, as to make the child like its parents. But the 
much greater variability, as well as the greater fre- 
quency of monstrosities, Under domestication or culti- 
vation, than under nature, leads me to believe that 
deviations of structure are in some way due to the 
nature of the conditions of life, to which the parents 
and their more remote ancestors have been exposed 
during several generations. I have remarked in the 
first chapter — but a long catalogue of facts which can- 
not be here given would be necessary to show the truth of 
the remark — that the reproductive system is eminently 
susceptible to changes in the conditions of life ; and to 


this system being functionally disturbed in the parents, 
I chiefly attribute the varying or plastic condition of 
the offspring. The male and female sexual elements 
seem to be affected before that union takes place which 
is to form a new being. In the case of "sporting" 
plants, the bud, which in its earliest condition does not 
apparently differ essentially from an ovule, is alone 
affected. But why, because the reproductive system is 
disturbed, this or that part should vary more or less, we 
are profoundly ignorant. Nevertheless, we can here 
and there dimly catch a faint ray of light, and we 
may feel sure that there must be some cause for each 
deviation of structure, however slight. 

How much direct effect difference of climate, food, 
&c, produces on any being is extremely doubtful. My 
impression is, that the effect is extremely small in the 
case of animals, but perhaps rather more in that of 
plants. We may, at least, safely conclude that such 
influences cannot have produced the many striking 
and complex co-adaptations of structure between one 
organic being and another, which we see everywhere 
throughout nature. Some little influence may be attri- 
buted to climate, food, &c. : thus, E. Forbes speaks 
confidently that shells at their southern limit, and when 
living in shallow water, are more brightly coloured than 
those of the same species further north or from greater 
depths. Gould believes that birds of the same species 
are more brightly coloured under a clear atmosphere, 
than when living on islands or near the coast. So with 
insects, Wollaston is convinced that residence near the 
sea affects their colours. Moquin-Tandon gives a list 
of plants which when growing near the sea-shore have 
their leaves in some degree fleshy, though not elsewhere 
fleshy. Several other such cases could be given. 

The fact of varieties of one species, when they range 


into the zone of habitation of other species, often 
acquiring in a very slight degree some of the characters 
of such species, accords with our view that species of 
all kinds are only well-marked and permanent varieties. 
Thus the species of shells which are confined to tropical 
and shallow seas are generally brighter-coloured than 
those confined to cold and deeper seas. The birds 
which are confined to continents are, according to Mr. 
Gould, brighter-coloured than those of islands. The 
insect-species confined to sea-coasts, as every collector 
knows, are often brassy or lurid. Plants which live 
exclusively on the sea-side are very apt to have fleshy 
leaves. He who believes in the creation of each spe- 
cies, will have to say that this shell, for instance, was 
created with bright colours for a warm sea ; but that 
this other shell became bright-coloured by variation 
when it ranged into warmer or shallower waters. 

When a variation is of the slightest use to a being, 
we cannot tell how much of it to attribute to the accu- 
mulative action of natural selection, and how much to 
the conditions of life. Thus, it is well known to fur- 
riers that animals of the same species have thicker and 
better fur the more severe the climate is under which 
they have lived ; but who can tell how much of this 
difference may be due to the warmest-clad individuals 
having been favoured and preserved during many 
generations, and how much to the direct action of 
the severe climate? for it would appear that climate 
has some direct action on the hair of our domestic 

Instances could be given of the same variety being 
produced under conditions of life as different as can 
well be conceived ; and, on the other hand, of different 
varieties being produced from the same species under 
the same conditions. Such facts show how indirectly 


the conditions of life must act. Again, innumerable 
instances are known to every naturalist of species 
keeping true, or not varying at all, although living 
under the most opposite climates. Such considerations 
as these incline me to lay very little weight on the 
direct action of the conditions of life. Indirectly, as 
already remarked, they seem to play an important part 
in affecting the reproductive system, and in thus in- 
ducing variability ; and natural selection will then accu- 
mulate all profitable variations, however slight, until 
they become plainly developed and appreciable by us. 

Effects of Use and Disuse. — From the facts alluded to 
in the first chapter, I think there can be little doubt 
that use in our domestic animals strengthens and en- 
larges certain parts, and disuse diminishes them ; and 
that such modifications are inherited. Under free 
nature, we can have no standard of comparison, by 
which to judge of the effects of long-continued use or 
disuse, for we know not the parent-forms; but many 
animals have structures which can be explained by the 
effects of disuse. As Professor Owen has remarked, there 
is no greater anomaly in nature than a bird that cannot 
fly ; yet there are several in this state. The logger- 
headed duck of South America can only flap along the 
surface of the water, and has its wings in nearly the 
same condition as the domestic Aylesbury duck. As the 
larger ground-feeding birds seldom take flight except to 
escape danger, I believe that the nearly wingless condi- 
tion of several birds, which now inhabit or have lately 
inhabited several oceanic islands, tenanted by no beast 
of prey, has been caused by disuse. The ostrich indeed 
inhabits continents and is exposed to danger from which 
it cannot escape by flight, but by kicking it can defend 
itself from enemies, as well as any of the smaller 

Chap. V. USE AND DISUSE. 135 

quadrupeds. We may imagine that the early progenitor 
of the ostrich had habits like those of a bustard, and 
that as natural selection increased in successive genera- 
tions the size and weight of its body, its legs were used 
more, and its wings less, until they became incapable 
of flight. 

Kirby has remarked (and I have observed the same 
fact) that the anterior tarsi, or feet, of many male dung- 
feeding beetles are very often broken off; he examined 
seventeen specimens in his own collection, and not one 
had even a relic left. In the Onites apelles the tarsi 
are so habitually lost, that the insect has been described 
as not having them. In some other genera they are pre- 
sent, but in a rudimentary condition. In the Ateuchus 
or sacred beetle of the Egyptians, they are totally defi- 
cient. There is not sufficient evidence to induce us 
to believe that mutilations are ever inherited ; and I 
should prefer explaining the entire absence of the ante- 
rior tarsi in Ateuchus, and their rudimentary condition 
in some other genera, by the long-continued effects of 
disuse in their progenitors ; for as the tarsi are almost 
always lost in many dung-feeding beetles, they must 
be lost early in life, and therefore cannot be much used 
by these insects. 

In some cases we might easily put down to disuse 
modifications of structure which are wholly, or mainly, 
due to natural selection. Mr. Wollaston has discovered 
the remarkable fact that 200 beetles, out of the 550 
species inhabiting Madeira, are so far deficient in wings 
that they cannot fly; and that of the twenty-nine 
endemic genera, no less than twenty-three genera have 
all their species in this condition ! Several facts, namely, 
that beetles in many parts of the world are very fre- 
quently blown to sea and perish ; that the beetles in 
Madeira, as observed by Mr. Wollaston, lie much con- 


cealed, until the wind lulls and the sun shines; that 
the proportion of wingless beetles is larger on the ex- 
posed Dezertas than in Madeira itself; and especially 
the extraordinary fact, so strongly insisted on by Mr. 
Wollaston, of the almost entire absence of certain large 
groups of beetles, elsewhere excessively numerous, and 
which groups have habits of life almost necessitating 
frequent flight ; — these several considerations have made 
me believe that the wingless condition of so many 
Madeira beetles is mainly due to the action of natural 
selection, but combined probably with disuse. For 
during thousands of successive generations each indi- 
vidual beetle which flew least, either from its wings 
having been ever so little less perfectly developed or 
from indolent habit, will have had the best chance of 
surviving from not being blown out to sea ; and, on the 
other hand, those beetles which most readily took to 
flight will oftenest have been blown to sea and thus 
have been destroyed. 

The insects in Madeira which are not ground-feeders, 
and which, as the flower-feeding coleoptera and lepidop- 
tera, must habitually use their wings to gain their subsist- 
ence, have, as Mr. Wollaston suspects, their wings not 
at all reduced, but even enlarged. This is quite com- 
patible with the action of natural selection. For when 
a new insect first arrived on the island, the tendency 
of natural selection to enlarge or to reduce the wings, 
would depend on whether a greater number of indivi- 
duals were saved by successfully battling with the winds, 
or by giving up the attempt and rarely or never flying. 
As with mariners shipwrecked near a coast, it would 
have been better for the good swimmers if they had 
been able to swim still further, whereas it would have 
been better for the bad swimmers if they had not been 
able to swim at all and had stuck to the wreck. 

Chap. V. USE AND DISUSE. 137 

The eyes of moles and of some burrowing rodents are 
rudimentary in size, and in some cases are quite covered 
up by skin and fur. This state of the eyes is probably 
due to gradual reduction from disuse, but aided perhaps 
by natural selection. In South America, a burrowing 
rodent, the tuco-tuco, or Ctenomys, is even more subter- 
ranean in its habits than the mole ; and I was assured 
by a Spaniard, who had often caught them, that they 
were frequently blind ; one which I kept alive was cer- 
tainly in this condition, the cause, as appeared on dis- 
section, having been inflammation of the nictitating 
membrane. As frequent inflammation of the eyes must 
be injurious to any animal, and as eyes are certainly 
not indispensable to animals with subterranean habits, 
a reduction in their size with the adhesion of the eye- 
lids and growth of fur over them, might in such case be 
an advantage ; and if so, natural selection would con- 
stantly aid the effects of disuse. 

It is well known that several animals, belonging to the 
most different classes, which inhabit the caves of Styria 
and of Kentucky, are blind. In some of the crabs the 
foot-stalk for the eye remains, though the eye is gone ; 
the stand for the telescope is there, though the telescope 
with its glasses has been lost. As it is difficult to ima- 
gine that eyes, though useless, could be in any way 
injurious to animals living in darkness, I attribute their 
loss wholly to disuse. In one of the blind animals, 
namely, the cave -rat, the eyes are of immense size ; and 
Professor Silliman thought that it regained, after living 
some days in the light, some slight power of vision. In 
the same manner as in Madeira the wings of some of 
the insects have been enlarged, and the wings of others 
have been reduced by natural selection aided by use 
and disuse, so in the case of the cave-rat natural selec- 
tion seems to have struggled with the loss of light and 


to have increased the size of the eyes; whereas with 
all the other inhabitants of the caves, disuse by itself 
seems to have done its work. 

It is difficult to imagine conditions of life more similar 
than deep limestone caverns under a nearly similar 
climate ; so that on the common view of the blind ani- 
mals having been separately created for the American 
and European caverns, close similarity in their organisa- 
tion and affinities might have been expected ; but, as 
Schiodte and others have remarked, this is not the case, 
and the cave-insects of the two continents are not more 
closely allied than might have been anticipated from the 
general resemblance of the other inhabitants of North 
America and Europe. On my view we must suppose 
that American animals, having ordinary powers of 
vision, slowly migrated by successive generations from 
the outer world into the deeper and deeper recesses of 
the Kentucky caves, as did European animals into the 
caves of Europe. We have some evidence of this gra- 
dation of habit ; for, as Schiodte remarks, " animals 
not far remote from ordinary forms, prepare the transi- 
tion from light to darkness. Next follow those that are 
constructed for twilight ; and, last of all, those destined 
for total darkness." By the time that an animal had 
reached, after numberless generations, the deepest re- 
cesses, disuse will on this view have more or less per- 
fectly obliterated its eyes, and natural selection will 
often have effected other changes, such as an increase 
in the length of the antennae or palpi, as a compensa- 
tion for blindness. Notwithstanding such modifications, 
we might expect still to see in the cave-animals of 
America, affinities to the other inhabitants of that con- 
tinent, and in those of Europe, to the inhabitants of 
the European continent. And this is the case with 
some of the American cave-animals, as I hear from 


Professor Dana; and some of the European cave- 
insects are very closely allied to those of the surround- 
ing country. It would be most difficult to give any 
rational explanation of the affinities of the blind cave- 
animals to the other inhabitants of the two continents 
on the ordinary view of their independent creation. 
That several of the inhabitants of the caves of the 
Old and New Worlds should be closely related, we 
might expect from the well-known relationship of most 
of their other productions. Far from feeling any sur- 
prise that some of the cave-animals should be very 
anomalous, as Agassiz has remarked in regard to the 
blind fish, the Amblyopsis, and as is the case with the 
blind Proteus with reference to the reptiles of Europe, 
I am only surprised that more wrecks of ancient life 
have not been preserved, owing to the less severe com- 
petition to which the inhabitants of these dark abodes 
will probably have been exposed. 

Acclimatisation. — Habit is hereditary with plants, as 
in the period of flowering, in the amount of rain requi- 
site for seeds to germinate, in the time of sleep, &c, 
and this leads me to say a few words on acclimatisa- 
tion. As it is extremely common for species of the 
same genus to inhabit very hot and very cold countries, 
and as I believe that all the species of the same genus 
have descended from a single parent, if this view be 
correct, acclimatisation must be readily effected during 
long-continued descent. It is notorious that each 
species is adapted to the climate of its own home : 
species from an arctic or even from a temperate region 
cannot endure a tropical climate, or conversely. So 
again, many succulent plants cannot endure a damp 
climate. But the degree of adaptation of species to 
the climates under which they live is often overrated. 


We may infer this from our frequent inability to pre- 
dict whether or not an imported plant will endure our 
climate, and from the number of plants and animals 
brought from warmer countries which here enjoy good 
health. We have reason to believe that species in a 
state of nature are limited in their ranges by the com- 
petition of other organic beings quite as much as, or 
more than, by adaptation to particular climates. But 
whether or not the adaptation be generally very close, we 
have evidence, in the case of some few plants, of their 
becoming, to a certain extent, naturally habituated to 
different temperatures, or becoming acclimatised : thus 
the pines and rhododendrons, raised from seed collected 
by Dr. Hooker from trees growing at different heights 
on the Himalaya, were found in this country to possess 
different constitutional powers of resisting cold. Mr. 
Thwaites informs me that he has observed similar facts 
in Ceylon, and analogous observations have been made 
by Mr. H. C. Watson on European species of plants 
brought from the Azores to England. In regard to 
animals, several authentic cases could be given of 
species within historical times having largely extended 
their range from warmer to cooler latitudes, and con- 
versely ; but we do not positively know that these ani- 
mals were strictly adapted to their native climate, but 
in all ordinary cases we assume such to be the case ; 
nor do we know that they have subsequently become 
acclimatised to their new homes. 

As I believe that our domestic animals were origin- 
ally chosen by uncivilised man because they were use- 
ful and bred readily under confinement, and not because 
they were subsequently found capable of far-extended 
transportation, I think the common and extraordinary 
capacity in our domestic animals of not only withstand- 
ing the most different climates but of being perfectly 


fertile (a far severer test) under them, may be used as 
an argument that a large proportion of other animals, 
now in a state of nature, could easily be brought to bear 
widely different climates. We must not, however, push 
the foregoing argument too far, on account of the pro- 
bable origin of some of our domestic animals from seve- 
ral wild stocks: the blood, for instance, of a tropical 
and arctic wolf or wild dog may perhaps be mingled in 
our domestic breeds. The rat and mouse cannot be 
considered as domestic animals, but they have been 
transported by man to many parts of the world, and 
now have a far wider range than any other rodent, 
living free under the cold climate of Faroe in the 
north and of the Falklands in the south, and on many 
islands in the torrid zones. Hence I am inclined to look 
at adaptation to any special climate as a quality readily 
grafted on an innate wide flexibility of constitution, 
which is common to most animals. On this view, the 
capacity of enduring the most different climates by man 
himself and by his domestic animals, and such facts as 
that former species of the elephant and rhinoceros were 
capable of enduring a glacial climate, whereas the liv- 
ing species are now all tropical or sub-tropical in their 
habits, ought not to be looked at as anomalies, but 
merely as examples of a very common .flexibility of con- 
stitution, brought, under peculiar circumstances, into 

How much of the acclimatisation of species to any 
peculiar climate is due to mere habit, and how much to 
the natural selection of varieties having different innate 
constitutions, and how much to both means combined, is 
a very obscure question. That habit or custom has some 
influence I must believe, both from analogy, and from 
the incessant advice given in agricultural works, even 
in the ancient Encyclopaedias of China, to be very cau- 


tious in transposing animals from one district to an- 
other; for it is not likely that man should have suc- 
ceeded in selecting so many breeds and sub-breeds with 
constitutions specially fitted for their own districts : the 
result must, I think, be due to habit. On the other 
hand, I can see no reason to doubt that natural selection 
will continually tend to preserve those individuals which 
are born with constitutions best adapted to their native 
countries. In treatises on many kinds of cultivated 
plants, certain varieties are said to withstand certain 
climates better than others : this is very strikingly 
shown in works on fruit trees published hi the United 
States, in which certain varieties are habitually recom- 
mended for the northern, and others for the southern 
States ; and as most of these varieties are of recent 
origin, they cannot owe their constitutional differences 
to habit. The case of the Jerusalem artichoke, which 
is never propagated by seed, and of which consequently 
new varieties have not been produced, has even been 
advanced — for it is now as tender as ever it was— as 
proving that acclimatisation cannot be effected! The 
case, also, of the kidney-bean has been often cited for a 
similar purpose, and with much greater weight ; but 
until some one will sow, during a score of generations, 
Ins kidney-beans so early that a very large proportion 
are destroyed by frost, and then collect seed from the 
few survivors, with care to prevent accidental crosses, 
and then again get seed from these seedlings, with the 
same precautions, the experiment cannot be said to 
have been even tried. Nor let it be supposed that no 
differences in the constitution of seedling kidney-beans 
ever appear, for an account has been published how 
much more hardy some seedlings appeared to be than 

On the whole, I think we may conclude that habit, 


use, and disuse, have, in some cases, played a consider- 
able part in the modification of the constitution, and 
of the structure of various organs ; but that the effects 
of use and disuse have often been largely combined 
with, and sometimes overmastered by, the natural selec- 
tion of innate differences. 

Correlation of Growth. — I mean by this expression 
that the whole organisation is so tied together during its 
growth and development, that when slight variations in 
any one part occur, and are accumulated through natural 
selection, other parts become modified. This is a very 
important subject, most imperfectly understood. The 
most obvious case is, that modifications accumulated 
solely for the good of the young or larva, will, it may 
safely be concluded, affect the structure of the adult ; 
in the same manner as any malconformation affecting 
the early embryo, seriously affects the whole organisa- 
tion of the adult. The several parts of the body which 
are homologous, and which, at an early embryonic period, 
are alike, seem liable to vary in an allied manner : we 
see this in the right and left sides of the body varying 
in the same manner ; in the front and hind legs, and 
even in the jaws and limbs, varying together, for the 
lower jaw is believed to be homologous with the limbs. 
These tendencies, I do not doubt, may be mastered 
more or less completely by natural selection: thus a 
family of stags once existed with an antler only on 
one side ; and if this had been of any great use to the 
breed it might probably have been rendered permanent 
by natural selection. 

Homologous parts, as has been remarked by some 
authors, tend to cohere ; this is often seen in monstrous 
plants ; and nothing is more common than the union of 
homologous parts in normal structures, as the union of 



the petals of the corolla into a tube. Hard parts seem 
to affect the form of adjoining soft parts ; it is believed 
by some authors that the diversity in the shape of the 
pelvis in birds causes the remarkable diversity in the 
shape of their kidneys. Others believe that the shape 
of the pelvis in the human mother influences by pres- 
sure the shape of the head of the child. In snakes, 
according to Schlegel, the shape of the body and the 
manner of swallowing determine the position of several 
of the most important viscera. 

The nature of the bond of correlation is very fre- 
quently quite obscure. M. Is. Geoffroy St. Hilaire has 
forcibly remarked, that certain malconformations very 
frequently, and that others rarely coexist, without our 
being able to assign any reason. What can be more 
singular than the relation between blue eyes and deaf- 
ness in cats, and the tortoise-shell colour with the female 
sex ; the feathered feet and skin between the outer toes 
in pigeons, and the presence of more or less down on the 
young birds when first hatched, with the future colour of 
their plumage ; or, again, the relation between the hair 
and teeth in the naked Turkish dog, though here pro- 
bably homology comes into play ? With respect to this 
latter case of correlation, I think it can hardly be acci- 
dental, that if we pick out the two orders of mammalia 
which are most abnormal in their dermal covering, viz. 
Cetacea (whales) and Edentata (armadilloes, scaly ant- 
eaters, &c), that these are likewise the most abnormal 
in their teeth. 

I know of no case better adapted to show the im- 
portance of the laws of correlation in modifying im- 
portant structures, independently of utility and, there- 
fore, of natural selection, than that of the difference 
between the outer and inner flowers in some Compo- 
sitous and Umbelliferous plants. Every one knows the 


difference in the ray and central florets of, for instance, 
the daisy, and this difference is often accompanied with 
the abortion of parts of the flower. But, in some Com- 
positous plants, the seeds also differ in shape and sculp- 
ture ; and even the ovary itself, with its accessory parts, 
differs, as has been described by Cassini. These differ- 
ences have been attributed by some authors to pressure, 
and the shape of the seeds in the ray-florets in some 
Compositse countenances this idea ; but, in the case of 
the corolla of the Umbellifera3, it is by no means, as Dr. 
Hooker informs me, in species with the densest heads 
that the inner and outer flowers most frequently differ. 
It might have been thought that the development of 
the ray-petals by drawing nourishment from certain 
other parts of the flower had caused their abortion ; but 
in some Composite there is a difference in the seeds of 
the outer and inner florets without any difference in the 
corolla. Possibly, these several differences may be con- 
nected with some difference in the flow of nutriment 
towards the central and external flowers: we know, 
at least, that in irregular flowers, those nearest to 
the axis are oftenest subject to peloria, and become 
regular. I may add, as an instance of this, and of a 
striking case of correlation, that I have recently observed 
in some garden pelargoniums, that the central flower of 
the truss often loses the patches of darker colour in the 
two upper petals ; and that when this occurs, the adhe- 
rent nectary is quite aborted ; when the colour is absent 
from only one of the two upper petals, the nectary is 
only much shortened. 

With respect to the difference in the corolla of the 
central and exterior flowers of a head or umbel, I do 
not feel at all sure that C. C. Sprengel's idea that the 
ray-florets serve to attract insects, whose agency is 
highly advantageous in the fertilisation of plants of 


these two orders, is so far-fetched, as it may at first 
appear: and if it be advantageous, natural selection 
may have come into play. But in regard to the differ- 
ences both in the internal and external structure of the 
seeds, which are not always correlated with any differ- 
ences in the flowers, it seems impossible that they can 
be in any way advantageous to the plant : yet in the 
Umbelliferae these differences are of such apparent im- 
portance — the seeds being in some cases, according to 
Tausch, orthospermous in the exterior flowers and coe- 
lospermous in the central flowers, — that the elder De 
Candolle founded his main divisions of the order on 
analogous differences. Hence we see that modifica- 
tions of structure, ( viewed by systematists as of high 
value, may be wholly due to unknown laws of correlated 
growth, and without being, as far as we can see, of the 
slightest service to the species. 

We may often falsely attribute to correlation of growth, 
structures which are common to whole groups of species, 
and which in truth are simply due to inheritance ; 
for an ancient progenitor may have acquired through 
natural selection some one modification in structure, 
and, after thousands of generations, some other and in- 
dependent modification ; and these two modifications, 
having been transmitted to a whole group of descendants 
with diverse habits, would naturally be thought to be 
correlated in some necessary manner. So, again, I do 
not doubt that some apparent correlations, occurring 
throughout whole orders, are entirely due to the manner 
. alone in which natural selection can act. For instance, 
' Alph. De Candolle has remarked that winged seeds are 
\ ^ never found in fruits which do not open : I should ex- 
plain the rule by the fact that seeds could not gradually 
become winged through natural selection, except in fruits 
which opened ; so that the individual plants producing 


seeds which were a little better fitted to be wafted 
further, might get an advantage over those producing 
seed less fitted for dispersal ; and this process could not 
possibly go on in fruit which did not open. 

The elder Geoffrey and Goethe propounded, at about 
the same period, their law of compensation or balan ce- 
ment of growth ; or, as Goethe expressed it, " in order 
to spend on one side, nature is forced to economise on 
the other side." I think this holds true to a certain ex- 
tent with our domestic productions : if nourishment flows 
to one part or organ in excess, it rarely flows, at least in 
excess, to another part ; thus it is difficult to get a cow 
to give much milk and to fatten readily. The same va- 
rieties of the cabbage do not yield abundant and nutri- 
tious foliage and a copious supply of oil-bearing seeds. 
When the seeds in our fruits become atrophied, the fruit 
itself gains largely in size and quality. In our poultry, 
a large tuft of feathers on the head is generally accom- 
panied by a diminished comb, and a large beard by 
diminished wattles. With species in a state of nature 
it can hardly be maintained that the law is of universal 
application ; but many good observers, more especially 
botanists, believe in its truth. I will not, however, here 
give any instances, for I see hardly any way of distin- 
guishing between the effects, on the one hand, of a part 
being largely developed through natural selection and 
another and adjoining part being reduced by tins same 
process or by disuse, and, on the other hand, the actual 
withdrawal of nutriment from one part owing to the 
excess of growth in another and adjoining part. 

I suspect, also, that some of the cases of compensation 
which have been advanced, and likewise some other 
facts, may be merged under a more general principle, 
namely, that natural selection is continually trying to 
economise in every part of the organisation. If under 



changed conditions of life a structure before useful be- 
comes less useful, any diminution, however slight, in its 
development, will be seized on by natural selection, for 
it will profit the individual not to have its nutriment 
wasted in building up an useless structure. I can thus 
only understand a fact with which I was much struck 
when examining cirripedes, and of which many other 
instances could be given : namely, that when a cirripede 
is parasitic within another and is thus protected, it loses 
more or less completely its own shell or carapace. This 
is the case with the male Ibla, and in a truly extraordi- 
nary manner with the Proteolepas : for the carapace in 
all other cirripedes consists of the three highly-important 
anterior segments of the head enormously developed, 
and furnished with great nerves and muscles; but in 
the parasitic and protected Proteolepas, the whole ante- 
rior part of the head is reduced to the merest rudiment 
attached to the bases of the prehensile antennae. Now 
the saving of a large and complex structure, when ren- 
dered superfluous by the parasitic habits of the Proteo- 
lepas, though effected by slow steps, would be a decided 
advantage to each successive individual of the species ; 
for in the struggle for life to which every animal is ex- 
posed, each individual Proteolepas would have a better 
chance of supporting itself, by less nutriment being 
wasted in developing a structure now become useless. 

Thus, as I believe, natural selection will always suc- 
ceed in the long run in reducing and saving every part 
of the organisation, as soon as it is rendered superfluous, 
without by any means causing some other part to be 
largely developed in a corresponding degree. And, con- 
versely, that natural selection may perfectly well suc- 
ceed in largely developing any organ, without requiring 
as a necessary compensation the reduction of some ad- 
joining part. 


It seems to be a rule, as remarked by Is. Geoffrey 
St. Hilaire, both in varieties and in species, that when 
any part or organ is repeated many times in the struc- 
ture of the same individual (as the vertebrae in snakes, 
and the stamens in polyandrous flowers) the number is 
variable ; whereas the number of the same part or organ, 
when it occurs in lesser numbers, is constant. The same 
author and some botanists have further remarked that 
multiple parts are also very liable to variation in struc- 
ture. Inasmuch as this " vegetative repetition," to use 
Prof. Owen's expression, seems to be a sign of low organi- 
sation ; the foregoing remark seems connected with the 
very general opinion of naturalists, that beings low in 
the scale of nature are more variable than those which 
are higher. I presume that lowness in this case means 
that the several parts of the organisation have been but 
little specialised for particular functions ; and as long as 
the same part has to perform diversified work, we can 
perhaps see why it should remain variable, that is, why 
natural selection should have preserved or rejected each 
little deviation of form less carefully than when the part 
has to serve for one special purpose alone. In the same 
way that a knife which has to cut all sorts of things 
may be of almost any shape ; whilst a tool for some 
particular object had better be of some particular shape. 
Natural selection, it should never be forgotten, can act 
on each part of each being, solely through and for its 

Kudimentary parts, it has been stated by some 
authors, and I believe with truth, are apt to be highly 
variable. We shall have to recur to the general subject 
of rudimentary and aborted organs ; and I will here only 
add that their variability seems to be owing to their 
uselessness, and therefore to natural selection having 
no power to check deviations in their structure. Thus 


rudimentary parts are left to the free play of the vari- 
ous laws of growth, to the effects of long-continued dis- 
use, and to the tendency to reversion. 

A part developed in any species in an extraordinary 
degree or manner, in comparison with the same part in 
allied species, tends to be highly variable. — Several years 
ago I was much struck with a remark, nearly to the 
above effect, published by Mr. Waterhouse. I infer 
also from an observation made by Professor Owen, with 
respect to the length of the arms of the ourang-outang, 
that he has come to a nearly similar conclusion. It is 
hopeless to attempt to convince any one of the truth of 
this proposition without giving the long array of facts 
which I have collected, and which cannot possibly be 
here introduced. I can only state my conviction that it is 
a rule of high generality. I am aware of several causes 
of error, but I hope that I have made due allowance for 
them. It should be understood that the rule by no 
means applies to any part, however unusually developed, 
unless it be unusually developed in comparison with 
the same part in closely allied species. Thus, the bat's 
wing is a most abnormal structure in the class mam- 
malia ; but the rule would not here apply, because there 
is a whole group of bats having wings; it would apply 
only if some one species of bat had its wings developed 
in some remarkable manner in comparison with the 
other species of the same genus. The rule applies very 
strongly in the case of secondary sexual characters, when 
displayed in any unusual manner. The term, secondary 
sexual characters, used by Hunter, applies to characters 
which are attached to one sex, but are not directly 
connected with the act of reproduction. The rule ap- 
plies to males and females ; but as females more rarely 
offer remarkable secondary sexual characters, it applies 


more rarely to them. The rule being so plainly appli- 
cable in the case of secondary sexual characters, may be 
due to the great variability of these characters, whether 
or not displayed in any unusual maimer — of which fact 
I think there can be little doubt. But that our rule is 
not confined to secondary sexual characters is clearly 
shown in the case of hermaphrodite cirripedes ; and 
I may here add, that I particularly attended to Mr. 
Waterhouse's remark, whilst investigating this Order, 
and I am fully convinced that the rule almost invari- 
ably holds good with cirripedes. I shall, in my future 
work, give a list of the more remarkable cases ; I will 
here only briefly give one, as it illustrates the rule in 
its largest application. The opercular valves of sessile 
cirripedes (rock barnacles) are, in every sense of the 
word, very important structures, and they differ ex- 
tremely little even in different genera ; but in the 
several species of one genus, Pyrgoma, these valves 
present a marvellous amount of diversification : the 
homologous valves in the different species being some- 
times wholly unlike in shape ; and the amount of varia- 
tion in the individuals of several of the species is 
so great, that it is no exaggeration to state that the 
varieties differ more from each other in the characters 
of these important valves than do other species of dis- 
tinct genera. 

As birds within the same country vary in a remark- 
ably small degree, I have particularly attended to them, 
and the rule seems to me certainly to hold good in this 
class. I cannot make out that it applies to plants, and 
this would seriously have shaken my belief in its truth, 
had not the great variability in plants made it particularly 
difficult to compare their relative degrees of variability. 

When we see any part or organ developed in a 
remarkable degree or manner in any species, the fair 


presumption is that it is of high importance to that 
species ; nevertheless the part in this case is eminently 
liable to variation. Why should this be so ? On the 
view that each species has been independently created, 
with all its parts as we now see them, I can see no 
explanation. But on the view that groups of species 
have descended from other species, and have been mo- 
dified through natural selection, I think we can obtain 
some light. In our domestic animals, if any part, or 
the whole animal, be neglected and no selection be ap- 
plied, that part (for instance, the comb in the Dorking 
fowl) or the whole breed will cease to have a nearly 
uniform character. The breed will then be said to have 
degenerated. In rudimentary organs, and in those 
which have been but little specialised for any particular 
purpose, and perhaps in polymorphic groups, we see a 
nearly parallel natural case ; for in such cases natural 
selection either has not or cannot come into full play, 
and thus the organisation is left in a fluctuating condi- 
tion. But what here more especially concerns us is, 
that in our domestic animals those points, which at the 
present time are undergoing rapid change by continued 
selection, are also eminently liable to variation. Look 
at the breeds of the pigeon ; see what a prodigious 
amount of difference there is in the beak of the differ- 
ent tumblers, in the beak and wattle of the different 
carriers, in the carriage and tail of our fantails, &c, 
these being the points now mainly attended to by Eng- 
lish fanciers. Even in the sub-breeds, as in the short- 
faced tumbler, it is notoriously difficult to breed them 
nearly to perfection, and frequently individuals are born 
which depart widely from the standard. There may be 
truly said to be a constant struggle going on between, 
on the one hand, the tendency to reversion to a less 
modified state, as well as an innate tendency to further 


variability of all kinds, and, on the other hand, the 
power of steady selection to keep the breed true. In 
the long run selection gains the day, and we do not 
expect to fail so far as to breed a bird as coarse as a 
common tumbler from a good short-faced strain. But 
as long as selection is rapidly going on, there may 
always be expected to be much variability in the struc- 
ture undergoing modification. It further deserves 
notice that these variable characters, produced by man's 
selection, sometimes become attached, from causes quite 
unknown to us, more to one sex than to the other, gene- 
rally to the male sex, as with the wattle of carriers and 
the enlarged crop of pouters. 

Now let us turn to nature. When a part has been 
developed in an extraordinary manner in any one 
species, compared with the other species of the same 
genus, we may conclude that this part has undergone 
an extraordinary amount of modification, since the 
period when the species branched off from the common 
progenitor of the genus. This period will seldom be 
remote in any extreme degree, as species very rarely 
endure for more than one geological period. An extra- 
ordinary amount of modification implies an unusually 
large and long-continued amount of variability, which 
has continually been accumulated by natural selection 
for the benefit of the species. But as the variability of 
the extraordinarily-developed part or organ has been so 
great and long-continued within a period not exces- 
sively remote, we might, as a general rule, expect still 
to find more variability in such parts than in other parts 
of the organisation, which have remained for a much 
longer period nearly constant. And this, I am con- 
vinced, is the case. That the struggle between natural 
selection on the one hand, and the tendency to rever- 
sion and variability on the other hand, will in the 



course of time cease ; and that the most abnormally 
developed organs may be made constant, I can see no 
reason to doubt. Hence when an organ, however 
abnormal it may be, has been transmitted in approxi- 
mately the same condition to many modified descend- 
ants, as in the case of the wing of the bat, it must 
have existed, according to my theory, for an immense 
period in nearly the same state; and thus it comes 
to be no more variable than any other structure. It 
is only in those cases in which the modification has 
been comparatively recent and extraordinarily great 
that we ought to find the generative variability, as it 
may be called, still present in a high degree. For in 
this case the variability will seldom as yet have been 
fixed by the continued selection of the individuals vary- 
ing in the required manner and degree, and by the con- 
tinued rejection of those tending to revert to a former 
and less modified condition. 

The principle included in these remarks may be 
extended. It is notorious that specific characters are 
more variable than generic. To explain by a simple 
example what is meant. If some species in a large genus 
of plants had blue flowers and some had red, the colour 
would be only a specific character, and no one would be 
surprised at one of the blue species varying into red, or 
conversely ; but if all the species had blue flowers, the 
colour would become a generic character, and its varia- 
tion would be a more unusual circumstance. I have 
chosen this example because an explanation is not in 
this case applicable, which most naturalists would ad- 
vance, namely, that specific characters are more variable 
than generic, because they are taken from parts of less 
physiological importance than those commonly used for 
classing genera. I believe this explanation is partly, 
yet only indirectly, true ; I shall, however, have to re- 


turn to this subject in our chapter on Classification. It 
would be almost superfluous to adduce evidence in 
support of the above statement, that specific characters 
are more variable than generic ; but I have repeatedly 
noticed in works on natural history, that w r hen an author 
has remarked with surprise that some important organ 
or part, which is generally very constant throughout 
large groups of species, has differed considerably in 
closely-allied species, that it has, also, been variable in 
the individuals of some of the species. And this fact 
shows that a character, which is generally of generic 
value, when it sinks in value and becomes only of spe- 
cific value, often becomes variable, though its physiolo- 
gical importance may remain the same. Something of 
the same kind applies to monstrosities : at least Is. Geof- 
froy St. Hilaire seems to entertain no doubt, that the 
more an organ normally differs in the different species 
of the same group, the more subject it is to individual 

On the ordinary view of each species having been 
independently created, w r hy should that part of the 
structure, w 7 hich differs from the same part in other 
independently-created species of the same genus, be more 
variable than those parts which are closely alike in the 
several species? I do not see that any explanation 
can be given. But on the view of species being only 
strongly marked and fixed varieties, we might surely 
expect to find them still often continuing to vary in those 
parts of their structure which have varied within a mode- 
rately recent period, and which have thus come to differ. 
Or to state the case in another manner : — the points in 
which all the species of a genus resemble each other, 
and in which they differ from the species of some other 
genus, are called generic characters ; and these charac- 
ters in common I attribute to inheritance from a common 


progenitor, for it can rarely have happened that natural 
selection will have modified several species, fitted to 
more or less widely-different habits, in exactly the same 
manner : and as these so-called generic characters have 
been inherited from a remote period, since that period 
when the species first branched off from their common 
progenitor, and subsequently have not varied or come to 
differ in any degree, or only in a slight degree, it is not 
probable that they should vary at the present day. On 
the other hand, the points in which species differ from 
other species of the same genus, are called specific cha- 
racters ; and as these specific characters have varied 
and come to differ within the period of the branching 
off of the species from a common progenitor, it is pro- 
bable that they should still often be in some degree 
variable, — at least more variable than those parts of 
the organisation which have for a very long period 
remained constant. 

In connexion with the present subject, I will make 
only two other remarks. I think it will be admitted, 
without my entering on details, that secondary sexual 
characters are very variable ; I think it also will be 
admitted that species of the same group differ from 
each other more widely in their secondary sexual cha- 
racters, than in other parts of their organisation ; com- 
pare, for instance, the amount of difference between the 
males of gallinaceous birds, in which secondary sexual 
characters are strongly displayed, with the amount of 
difference between their females ; and the truth of this 
proposition will be granted. The cause of the original 
variability of secondary sexual characters is not mani- 
fest ; but we can see why these characters should not 
have been rendered as constant and uniform as other 
parts of the organisation ; for secondary sexual charac- 
ters have been accumulated by sexual selection, which 


is less rigid in its action than ordinary selection, as it 
does not entail death, but only gives fewer offspring to 
the less favoured males. Whatever the cause may be 
of the variability of secondary sexual characters, as they 
are highly variable, sexual selection will have had a 
wide scope for action, and may thus readily have suc- 
ceeded in giving to the species of the same group a 
greater amount of difference in their sexual characters, 
than in other parts of their structure. 

It is a remarkable fact, that the secondary sexual 
differences between the two sexes of the same species 
are generally displayed in the very same parts of the 
organisation in which the different species of the same 
genus differ from each other. Of this fact I will give 
in illustration two instances, the first which happen 
to stand on my list; and as the differences in these 
cases are of a very unusual nature, the relation can 
hardly be accidental. The same number of joints 
in the tarsi is a character generally common to very 
large groups of beetles, but in the Engidse, as West wood 
has remarked, the number varies greatly ; and the 
number likewise differs in the two sexes of the same 
species : again in fossorial hymenoptera, the manner of 
neuration of the wings is a character of the Inghest 
importance, because common to large groups ; but in 
certain genera the neuration differs in the different spe- 
cies, and likewise in the two sexes of the same species. 
This relation has a clear meaning on my view of the 
subject : I look at all the species of the same genus as 
having as certainly descended from the same progenitor, 
as have the two sexes of any one of the species. Con- 
sequently, whatever part of the structure of the common 
progenitor, or of its early descendants, became variable ; 
variations of this part would, it is highly probable, be 
taken advantage of by natural and sexual selection, in 


order to fit the several species to their several places 
in the economy of nature, and likewise to fit the two 
sexes of the same species to each other, or to fit the 
males and females to different habits of life, or the 
males to struggle with other males for the possession 
of the females. 

Finally, then, I conclude that the greater variability 
of specific characters, or those which distinguish species 
from species, than of generic characters, or those which 
the species possess in common; — that the frequent ex- 
treme variability of any part which is developed in a 
species in an extraordinary manner in comparison with 
the same part in its congeners ; and the not great 
degree of variability in a part, however extraordinarily 
it may be developed, if it be common to a whole group 
of species ; — that the great variability of secondary 
sexual characters, and the great amount of difference in 
these same characters between closely allied species ; — 
that secondary sexual and ordinary specific differences 
are generally displayed in the same parts of the organ- 
isation, — are all principles closely connected together. 
All being mainly due to the species of the same group 
having descended from a common progenitor, from 
whom they have inherited much in common, — to parts 
which have recently and largely varied being more 
likely still to go on varying than parts which have long 
been inherited and have not varied, — to natural selec- 
tion having more or less completely, according to the 
lapse of time, overmastered the tendency to reversion 
and to further variability, — to sexual selection being 
less rigid than ordinary selection, — and to variations in 
the same parts having been accumulated by natural 
and sexual selection, and thus adapted for secondary 
sexual, and for ordinary specific purposes. 


Distinct species present analogous variations; and a 
variety of one species often assumes some of the characters 
of an allied species, or reverts to some of the characters of 
an early progenitor. — These propositions will be most 
readily understood by looking to our domestic races. 
The most distinct breeds of pigeons, in countries most 
widely apart, present sub-varieties with reversed feathers 
on the head and feathers on the feet, — characters not 
possessed by the aboriginal rock-pigeon ; these then 
are analogous variations in two or more distinct races. 
The frequent presence of fourteen or even sixteen tail- 
feathers in the pouter, may be considered as a variation 
representing the normal structure of another race, the 
fantail. I presume that no one will doubt that all such 
analogous variations are due to the several races of 
the pigeon having inherited from a common parent the 
same constitution and tendency to variation, when acted 
on by similar unknown influences. In the vegetable 
kingdom we have a case of analogous variation, in the 
enlarged stems, or roots as commonly called, of the 
Swedish turnip and Kuta baga, plants which several 
botanists rank as varieties produced by cultivation from 
a common parent : if this be not so, the case will then 
be one of analogous variation in two so-called distinct 
species ; and to these a third may be added, namely, 
the common turnip. According to the ordinary view 
of each species having been independently created, 
we should have to attribute this similarity in the en- 
larged stems of these three plants, not to the vera causa 
of community of descent, and a consequent tendency 
to vary in a like manner, but to three separate yet 
closely related acts of creation. 

"With pigeons, however, we have another case, namely, 
the occasional appearance in all the breeds, of slaty- 
blue birds with two black bars on the wings, a white 


rump, a bar at the end of the tail, with the outer 
feathers externally edged near their bases with white. 
As all these marks are characteristic of the parent rock- 
pigeon, I presume that no one will doubt that this is 
a case of reversion, and not of a new yet analogous 
variation appearing in the several breeds. We may I 
think confidently come to this conclusion, because, as 
we have seen, these coloured marks are eminently liable 
to appear in the crossed offspring of two distinct and 
differently coloured breeds ; and in this case there is 
nothing in the external conditions of life to cause the 
reappearance of the slaty-blue, with the several marks, 
beyond the influence of the mere act of crossing on the 
laws of inheritance. 

No doubt it is a very surprising fact that characters 
should reappear after having been lost for many, perhaps 
for hundreds of generations. But when a breed has 
been crossed only once by some other breed, the offspring 
occasionally show a tendency to revert in character to 
the foreign breed for many generations — some say, for 
a dozen or even a score of generations. After twelve 
generations, the proportion of blood, to use a common 
expression, of any one ancestor, is only 1 in 2048 ; and 
yet, as we see, it is generally believed that a tendency 
to reversion is retained by this very small proportion of 
foreign blood. In a breed which has not been crossed, 
but in which both parents have lost some character 
which their progenitor possessed, the tendency, whether 
strong or weak, to reproduce the lost character might 
be, as was formerly remarked, for all that we can see 
to the contrary, transmitted for almost any number of 
generations. When a character which has been lost in 
a breed, reappears after a great number of generations, 
the most probable hypothesis is, not that the offspring sud- 
denly takes after an ancestor some hundred generations 

Chap.V. laws of vakiatton. 161 

distant, but that in each successive generation there has 
been a tendency to reproduce the character in question, 
which at last, under unknown favourable conditions, 
gains an ascendancy. For instance, it is probable that 
in each generation of the barb-pigeon, which produces 
most rarely a blue and black-barred bird, there has 
been a tendency in each generation in the plumage to 
assume this colour. This view is hypothetical, but could 
be supported by some facts; and I can see no more 
abstract improbability in a tendency to produce any cha- 
racter being inherited for an endless number of genera- 
tions, than in quite useless or rudimentary organs being, 
as we all know them to be, thus inherited. Indeed, we 
may sometimes observe a mere tendency to produce a 
rudiment inherited : for instance, in the common snap- 
dragon (Antirrhinum) a rudiment of a fifth stamen so 
often appears, that this plant must have an inherited 
tendency to produce it. 

As all the species of the same genus are supposed, 
on my theory, to have descended from a common parent, 
it might be expected that they would occasionally vary 
in an analogous manner ; so that a variety of one species 
would resemble in some of its characters another species ; 
this other species being on my view only a well-marked 
and permanent variety. But characters thus gained 
would probably be of an unimportant nature, for the 
presence of all important characters will be governed 
by natural selection, in accordance with the diverse 
habits of the species, and will not be left to the mutual 
action of the conditions of life and of a similar in- 
herited constitution. It might further be expected 
that the species of the same genus would occasionally 
exhibit reversions to lost ancestral characters. As, how- 
ever, we never know the exact character of the common 
ancestor of a group, we could not distinguish these two 


cases : if, for instance, we did not know that the rock- 
pigeon was not feather-footed or turn-crowned, we could 
not have told, whether these characters in our domestic 
breeds were reversions or only analogous variations ; but 
we might have inferred that the blueness was a case of 
reversion, from the number of the markings, which are 
correlated with the blue tint, and which it does not ap- 
pear probable would all appear together from simple 
variation. More especially we might have inferred this, 
from the blue colour and marks so often appearing 
when distinct breeds of diverse colours are crossed. 
Hence, though under nature it must generally be left 
doubtful, what cases are reversions to an anciently ex- 
isting character, and what are new but analogous varia- 
tions, yet we ought, on my theory, sometimes to find 
the varying offspring of a species assuming characters 
(either from reversion or from analogous variation) 
which already occur in some other members of the 
same group. And this undoubtedly is the case in 

A considerable part of the difficulty in recognising a 
variable species in our systematic works, is due to its 
varieties mocking, as it were, some of the other spe- 
cies of the same genus. A considerable catalogue, also, 
could be given of forms intermediate between two other 
forms, which themselves must be doubtfully ranked as 
either varieties or species ; and this shows, unless all 
these forms be considered as independently created 
species, that the one in varying has assumed some of 
the characters of the other, so as to produce the inter- 
mediate form. But the best evidence is afforded by 
parts or organs of an important and uniform nature 
occasionally varying so as to acquire, in some degree, 
the character of the same part or organ in an allied 
species. I have collected a long list of such cases ; but 


here, as before, I lie under a great disadvantage in 
not being able to give them. I can only repeat that 
such cases certainly do occur, and seem to me very 

I will, however, give one curious and complex case, 
not indeed as affecting any important character, but 
from occurring in several species of the same genus, 
partly under domestication and partly under nature. 
It is a case apparently of reversion. The ass not rarely 
has very distinct transverse bars on its legs, like those 
on the legs of the zebra : it has been asserted that these 
are plainest in the foal, and from inquiries which I have 
made, I believe this to be true. It has also been as- 
serted that the stripe on each shoulder is sometimes 
double. The shoulder-stripe is certainly very variable 
in length and outline. A white ass, but not an albino, 
has been described without either spinal or shoulder 
stripe ; and these stripes are sometimes very obscure, or 
actually quite lost, in dark-coloured asses. The koulan 
of Pallas is said to have been seen with a double shoulder- 
stripe. The hemionus has no shoulder-stripe ; but traces 
of it, as stated by Mr. Blyth and others, occasionally 
appear: and I have been informed by Colonel Poole 
that the foals of this species are generally striped on 
the legs, and faintly on the shoulder. The quagga, 
though so plainly barred like a zebra over the body, is 
without bars on the legs ; but Dr. Gray has figured 
one specimen with very distinct zebra-like bars on the 

With respect to the horse, I have collected cases in 
England of the spinal stripe in horses of the most dis- 
tinct breeds, and of all colours ; transverse bars on the 
legs are not rare in duns, mouse-duns, and in one 
instance in a chestnut : a faint shoulder-stripe may 
sometimes be seen in duns, and I have seen a trace in a 

164 LAWS OF VAK1ATION. Chap. V. 

bay horse. My son made a careful examination and 
sketch for me of a dun Belgian cart-horse with a double 
stripe on each shoulder and with leg-stripes ; and a man, 
whom I can implicitly trust, has examined for me a 
small dun Welch pony with three short parallel stripes 
on each shoulder. 

In the north-west part of India the Kattywar breed 
of horses is so generally striped, that, as I hear from 
Colonel Poole, who examined the breed for the Indian 
Government, a horse without stripes is not considered as 
purely-bred. The spine is always striped ; the legs are 
generally barred ; and the shoulder-stripe, winch is some- 
times double and sometimes treble, is common ; the side 
of the face, moreover, is sometimes striped. The stripes 
are plainest in the foal ; and sometimes quite disappear 
in old horses. Colonel Poole has seen both gray and 
bay Kattywar horses striped when first foaled. I 
have, also, reason to suspect, from information given 
me by Mr. W. W. Edwards, that with the English race- 
horse the spinal stripe is much commoner in the foal 
than in the full-grown animal. Without here entering 
on further details, I may state that I have collected cases 
of leg and shoulder stripes in horses of very different 
breeds, in various countries from Britain to Eastern 
China; and from Norway in the north to the Malay 
Archipelago in the south. In all parts of the world 
these stripes occur far oftenest in duns and mouse-duns ; 
by the term dun a large range of colour is included, 
from one between brown and black to a close approach 
to cream-colour. 

I am aware that Colonel Hamilton Smith, who has 
written on this subject, believes that the several breeds 
of the horse have descended from several aboriginal 
species — one of which, the dun, was striped ; and that 
the above-described appearances are all due to ancient 


crosses with the dun stock. But I am not at all satisfied 
with this theory, and should be loth to apply it to breeds 
so distinct as the heavy Belgian cart-horse, Welch ponies, 
cobs, the lanky Kattywar race, &c, inhabiting the most 
distant parts of the world. 

Now let us turn to the effects of crossing the several 
species of the horse-genus. Kollin asserts, that the 
common mule from the ass and horse is particularly apt 
to have bars on its legs. I once saw a mule with 
its legs so much striped that any one at first would 
have thought that it must have been the product of a 
zebra ; and Mr. W. C. Martin, in his excellent treatise 
on the horse, has given a figure of a similar mule. In 
four coloured drawings, which I have seen, of hybrids 
between the ass and zebra, the legs were much more 
plainly barred than the rest of the body; and in one 
of them there was a double shoulder-stripe. In Lord 
Moreton's famous hybrid from a chestnut mare and male 
quagga, the hybrid, and even the pure offspring sub- 
sequently produced from the mare by a black Arabian 
sire, were much more plainly barred across the legs than 
is even the pure quagga. Lastly, and this is another 
most remarkable case, a hybrid has been figured by 
Dr. Gray (and he informs me that he knows of a second 
case) from the ass and the hemionus ; and this hybrid, 
though the ass seldom has stripes on its legs and the 
hemionus has none and has not even a shoulder-stripe, 
nevertheless had all four legs barred, and had three 
short shoulder-stripes, like those on the dun Welch 
pony, and even had some zebra-like stripes on the 
sides of its face. With respect to this last fact, I 
was so convinced that not even a stripe of colour 
appears from what would commonly be called an acci- 
dent, that I was led solely from the occurrence of the 
face-stripes on this hybrid from the ass and hemionus, 


to ask Colonel Poole whether such face-stripes ever 
occur in the eminently striped Katty war breed of horses, 
and was, as we have seen, answered in the affirmative. 

What now are we to say to these several facts ? We 
see several very distinct species of the horse-genus 
becoming, by simple variation, striped on the legs like 
a zebra, or striped on the shoulders like an ass. In the 
horse we see this tendency strong whenever a dun tint 
appears — a tint which approaches to that of the general 
colouring of the other species of the genus. The 
appearance of the stripes is not accompanied by any 
change of form or by any other new character. We see 
this tendency to become striped most strongly displayed 
in hybrids from between several of the most distinct 
species. Now observe the case of the several breeds of 
pigeons : they are descended from a pigeon (including 
two or three sub-species or geographical races) of a 
bluish colour, with certain bars and other marks ; and 
when any breed assumes by simple variation a bluish 
tint, these bars and other marks invariably reappear ; 
but without any other change of form or character. 
When the oldest and truest breeds of various colours 
are crossed, we see a strong tendency for the blue tint 
and bars and marks to reappear in the mongrels. I 
have stated that the most probable hypothesis to account 
for the reappearance of very ancient characters, is — 
that there is a tendency in the young of each successive 
generation to produce the long-lost character, and that 
this tendency, from unknown causes, sometimes prevails. 
And we have just seen that in several species of the 
horse-genus the stripes are either plainer or appear 
/ more commonly in the young than in the old. Call 
the breeds of pigeons, some of which have bred true 
for centuries, species ; and how exactly parallel is 
! the case with that of the species of the horse-genus ! 


Chap. V. SUMMARY. 167 

For myself, I venture confidently to look back thousands 
on thousands of generations, and I see an animal striped 
like a zebra, but perhaps otherwise very differently 
constructed, the common parent of our domestic horse, 
whether or not it be descended from one or more wild 
stocks, of the ass, the hemionus, quagga, and zebra. 

He who believes that each equine species was inde- 
pendently created, will, I presume, assert that each 
species has been created with a tendency to vary, both 
under nature and under domestication, in this par- 
ticular manner, so as often to become striped like 
other species of the genus ; and that each has been 
created with a strong tendency, when crossed with species 
inhabiting distant quarters of the world, to produce 
hybrids resembling in their stripes, not their own / / 
parents, but other species of the genus. To admit 
this view is, as it seems to me, to reject a real for an ' 
unreal, or at least for an unknown, cause. It makes 
the works of God a mere mockery and deception ; I 
would almost as soon believe with the old and ignorant 
cosmogonists, that fossil shells had never lived, but had 
been created in stone so as to mock the shells now 
living on the sea-shore. 

Summary. — Our ignorance of the laws of variation is 
profound. Not in one case out of a hundred can we 
pretend to assign any reason why this or that part 
differs, more or less, from the same part in the parents. 
But whenever we have the means of instituting a com- 
parison, the same laws appear to have acted in pro- 
ducing the lesser differences between varieties of the 
same species, and the greater differences between species 
of the same genus. The external conditions of life, as 
climate and food, &c, seem to have induced some slight 
modifications. Habit in producing constitutional dif- 


ferences, and use in strengthening, and disuse in weak- 
ening and diminishing organs, seem to have been more 
potent in their effects. Homologous parts tend to vary 
in the same way, and homologous parts tend to cohere. 
Modifications in hard parts and in external parts some- 
times affect softer and internal parts. When one part 
is largely developed, perhaps it tends to draw nourish- 
ment from the adjoining parts ; and every part of the 
structure which can be saved without detriment to the 
individual, will be saved. Changes of structure at an 
early age will generally affect parts subsequently de- 
veloped ; and there are very many other correlations of 
growth, the nature of which we are utterly unable to 
understand. Multiple parts are variable in number and 
in structure, perhaps arising from such parts not having 
been closely specialised to any particular function, so 
that their modifications have not been closely checked 
by natural selection. It is probably from this same 
cause that organic beings low in the scale of nature are 
more variable than those which have their whole organi- 
sation more specialised, and are higher in the scale. 
Rudimentary organs, from being useless, will be disre- 
garded by natural selection, and hence probably are 
variable. Specific characters — that is, the characters 
which have come to differ since the several species of 
the same genus branched off from a common parent — 
are more variable than generic characters, or those 
which have long been inherited, and have not differed 
within this same period. In these remarks we have 
referred to special parts or organs being still variable, 
because they have recently varied and thus come to 
differ; but we have also seen in the second Chapter 
that the same principle applies to the whole individual ; 
for in a district where many species of any genus are 
found — that is, where there has been much former 

Chap. V. SUMMARY. 169 

variation and differentiation, or where the manufactory 
of new specific forms has been actively at work — there, 
on an average, we now find most varieties or incipient 
species. Secondary sexual characters are highly vari- 
able, and such characters differ much in the species 
of the same group. Variability in the same parts of 
the organisation has generally been taken advantage 
of in giving secondary sexual differences to the sexes 
of the same species, and specific differences to the 
several species of the same genus. Any part or organ 
developed to an extraordinary size or in an extraor- 
dinary manner, in comparison with the same part or 
organ in the allied species, must have gone through an 
extraordinary amount of modification since the genus 
arose ; and thus we can understand why it should often 
still be variable in a much higher degree than other 
parts ; for variation is a long-continued and slow pro- 
cess, and natural selection will in such cases not as 
yet have had time to overcome the tendency to further 
variability and to reversion to a less modified state. But 
when a species with any extraordinarily-developed organ 
has become the parent of many modified descendants 
— which on my view must be a very slow process, 
requiring a long lapse of time — iu this case, natural 
selection may readily have succeeded in giving a fixed 
character to the organ, in however extraordinary a 
manner it may be developed. Species inheriting nearly 
the same constitution from a common parent and ex- 
posed to similar influences will naturally tend to present 
analogous variations, and these same species may occa- 
sionally revert to some of the characters of their ancient 
progenitors. Although new and important modifica- 
tions may not arise from reversion and analogous varia- 
tion, such modifications will add to the beautiful and 
harmonious diversity of nature. 


Whatever the cause may be of each slight difference 
in the offspring from their parents — and a cause for 
each must exist — it is the steady accumulation, through 
natural selection, of such differences, when beneficial to 
the individual, that gives rise to all the more important 
modifications of structure, by which the innumerable 
beings on the face of this earth are enabled to struggle 
with each other, and the best adapted to survive. 




Difficulties on the theory of descent with modification — Transitions — 
Absence or rarity of transitional varieties — Transitions in habits 
of life — Diversified habits in the same species — Species with 
habits widely different from those of their allies — Organs of 
extreme perfection — Means of transition — Cases of difficulty — 
Natura non facit saltum — Organs of small importance — Organs 
not in all cases absolutely perfect — The law of Unity of Type 
and of the Conditions of Existence embraced by the theory of 
Natural Selection. 

Long before having arrived at this part of my work, a 
crowd of difficulties will have occurred to the reader. 
Some of them are so grave that to this day I can never 
reflect on them without being staggered ; but, to the best 
of my judgment, the greater number are only apparent, 
and those that are real are not, I think, fatal to my 

These difficulties and objections may be classed under 
the following heads : — Firstly, why, if species have 
descended from other species by insensibly fine grada- 
tions, do we not everywhere see innumerable transitional 
forms ? Why is not all nature in confusion instead of 
the species being, as we see them, well defined ? 

Secondly, is it possible that an animal having, for 
instance, the structure and habits of a bat, could have 
been formed by the modification of some animal with 
wholly different habits ? Can we believe that natural 
selection could produce, on the one hand, organs of 
trifling importance, such as the tail of a giraffe, which 
serves as a fly-flapper, and, on the other hand, organs of 



such wonderful structure, as the eye, of which we hardly 
as yet fully understand the inimitable perfection ? 

Thirdly, can instincts be acquired and modified through 
natural selection ? What shall we say to so marvellous 
an instinct as that which leads the bee to make cells, 
which have practically anticipated the discoveries of 
profound mathematicians ? 

Fourthly, how can we accoimt for species, when crossed, 
being sterile and producing sterile offspring, whereas, 
when varieties are crossed, their fertility is unim- 
paired ? 

The two first heads shall be here discussed — Instinct 
and Hybridism in separate chapters. 

On the absence or rarity of transitional varieties. — 
As natural selection acts solely by the preservation of 
profitable modifications, each new form will tend in a 
fully-stocked country to take the place of, and finally to 
exterminate, its own less improved parent or other less- 
favoured forms with which it comes into competition. 
Thus extinction and natural selection will, as we have 
seen, go hand in hand. Hence, if we look at each species 
as descended from some other unknown form, botli the 
parent and all the transitional varieties will generally 
have been exterminated by the very process of forma- 
tion and perfection of the new form. 

But, as by this theory innumerable transitional forms 
must have existed, why do we not find them embedded 
in countless numbers in the crust of the earth ? It will 
be much more convenient to discuss this question in the 
chapter on the Imperfection of the geological record ; 
and I will here only state that I believe the answer 
mainly lies in the record being incomparably less perfect 
than is generally supposed ; the imperfection of the 
record being chiefly due to organic beings not inhabiting 


profound depths of the sea, and to their remains being 
embedded and preserved to a future age only in masses 
of sediment sufficiently thick and extensive to withstand 
an enormous amount of future degradation ; and such 
fossiliferous masses can be accumulated only where much 
sediment is deposited on the shallow bed of the sea, 
whilst it slowly subsides. These contingencies will 
concur only rarely, and after enormously long intervals. 
Whilst the bed of the sea is stationary or is rising, or 
when very little sediment is being deposited, there will 
be blanks in our geological history. The crust of the 
earth is a vast museum; but the natural collections 
have been made only at intervals of time immensely 

But it may be urged that when several closely-allied 
species inhabit the same territory we surely ought to 
find at the present time many transitional forms. Let 
us take a simple case : in travelling from north to south 
over a continent, we generally meet at successive inter- 
vals with closely allied or representative species, evi- 
dently filling nearly the same place in the natural 
economy of the land. These representative species 
often meet and interlock; and as the one becomes 
rarer and rarer, the other becomes more and more fre- 
quent, till the one replaces the other. But if we com- 
pare these species where they intermingle, they are 
generally as absolutely distinct from each other in every 
detail of structure as are specimens taken from the 
metropolis inhabited by each. By my theory these 
allied species have descended from a common parent; 
and during the process of modification, each has become 
adapted to the conditions of life of its own region, and 
has supplanted and exterminated its original parent 
and all the transitional varieties between its past and 
present states. Hence we ought not to expect at the 


present time to meet with numerous transitional vari- 
eties in each region, though they must have existed 
there, and may be embedded there in a fossil condition. 
But in the intermediate region, having intermediate 
conditions of life, why do we not now find closely-linking 
intermediate varieties ? This difficulty for a long time 
quite confounded me. But I think it can be in large 
part explained. 

In the first place we should be extremely cautious 
in inferring, because an area is now continuous, that it 
has been continuous during a long period. Geology 
would lead us to believe that almost every continent 
has been broken up into islands even during the later 
tertiary periods ; and in such islands distinct species 
might have been separately formed without the possi- 
bility of intermediate varieties existing in the interme- 
diate zones. By changes in the form of the land and 
of climate, marine areas now continuous must often 
have existed within recent times in a far less continuous 
and uniform condition than at present. But I will pass 
over this way of escaping from the difficulty ; for I 
believe that many perfectly defined species have been 
formed on strictly continuous areas ; though I do not 
doubt that the formerly broken condition of areas now 
continuous has played an important part in the forma- 
tion of new species, more especially with freely-crossing 
and wandering animals. 

In looking at species as they are now distributed 
over a wide area, we generally find them tolerably 
numerous over a large territory, then becoming some- 
what abruptly rarer and rarer on the confines, and 
finally disappearing. Hence the neutral territory be- 
tween two representative species is generally narrow in 
comparison with the territory proper to each. We see 
the same fact in ascending mountains, and sometimes 


it is quite remarkable how abruptly, as Alph. De Can- 
clolle has observed, a common alpine species disap- 
pears. The same fact has been noticed by Forbes in 
sounding the depths of the sea with the dredge. To 
those who look at climate and the physical conditions of 
life as the all-important elements of distribution, these 
facts ought to cause surprise, as climate and height or 
depth graduate away insensibly. But when we bear in 
mind that almost every species, even in its metropolis, 
would increase immensely in numbers, were it not for 
other competing species ; that nearly all either prey on 
or serve as prey for others ; in short, that each organic 
being is either directly or indirectly related in the most 
important manner to other organic beings, we must see 
that the range of the inhabitants of any country by 
no means exclusively depends on insensibly changing 
physical conditions, but in large part on the presence of 
other species, on which it depends, or by which it is 
destroyed, or with which it comes into competition ; and 
as these species are already denned objects (however 
they may have become so), not blending one into another 
by insensible gradations, the range of any one species, 
depending as it does on the range of others, will tend to 
be sharply denned. Moreover, each species on the con- 
fines of its range, where it exists in lessened numbers, 
will, during fluctuations in the number of its enemies 
or of its prey, or in the seasons, be extremely liable 
to utter extermination ; and thus its geographical range 
will come to be still more sharply defined. 

If I am right in believing that allied or representative 
species, when inhabiting a continuous area, are gene- 
rally so distributed that each has a wide range, with 
a comparatively narrow neutral territory between them, 
in which they become rather suddenly rarer and rarer ; 
then, as varieties do not essentially differ from species, 


the same rule will probably apply to both ; and if we in 
imagination adapt a varying species to a very large area, 
we shall have to adapt two varieties to two large areas, 
and a third variety to a narrow intermediate zone. The 
intermediate variety, consequently, will exist in lesser 
numbers from inhabiting a narrow and lesser area ; and 
practically, as far as I can make out, this rule holds good 
with varieties in a state of nature. I have met with 
striking instances of the rule in the case of varieties 
intermediate between well-marked varieties in the genus 
Balanus. And it would appear from information given 
me by Mr. Watson, Dr. Asa Gray, and Mr. Wollaston, 
that generally when varieties intermediate between 
two other forms occur, they are much rarer numerically 
than the forms which they connect. Now, if we may 
trust these facts and inferences, and therefore conclude 
that varieties linking two other varieties together have 
generally existed in lesser numbers than the forms which 
they connect, then, I think, we can understand why in- 
termediate varieties should not endure for very long 
periods ; — why as a general rule they should be exter- 
minated and disappear, sooner than the forms which 
they originally linked together 

For any form existing in lesser numbers would, as 
already remarked, run a greater chance of being exter- 
minated than one existing in large numbers ; and in 
this particular case the intermediate form would be 
eminently liable to the inroads of closely allied forms 
existing on both sides of it. But a far more important 
consideration, as I believe, is that, during the process 
of further modification, by which two varieties are sup- 
posed on my theory to be converted and perfected into 
two distinct species, the two which exist in larger 
numbers from inhabiting larger areas, will have a great 
advantage over the intermediate variety, which exists 


in smaller numbers in a narrow and intermediate zone. 
For forms existing in larger numbers will always have 
a better chance, within any given period, of presenting 
further favourable variations for natural selection to 
seize on, than will the rarer forms which exist in lesser 
numbers. Hence, the more common forms, in the race 
for life, will tend to beat and supplant the less common 
forms, for these will be more slowly modified and im- 
proved. It is the same principle which, as I believe, 
accounts for the common species in each country, as 
shown in the second chapter, presenting on an average 
a greater number of well-marked varieties than do the 
rarer species. I may illustrate what I mean by sup- 
posing three varieties of sheep to be kept, one adapted 
to an extensive mountainous region ; a second to a com- 
paratively narrow, hilly tract ; and a third to wide 
plains at the base; and that the inhabitants are all 
trying with equal steadiness and skill to improve their 
stocks by selection ; the chances in this case will be 
strongly in favour of the great holders on the moun- 
tains or on the plains improving their breeds more 
quickly than the small holders on the intermediate 
narrow, hilly tract ; and consequently the improved 
mountain or plain breed will soon take the place of 
the less improved hill breed ; and thus the two breeds, 
which originally existed in greater numbers, will come 
into close contact with each other, without the inter- 
position of the supplanted, intermediate hill-variety. 

To sum up, I believe that species come to be tolerably 
well-defined objects, and do not at any one period pre- 
sent an inextricable chaos of varying and intermediate 
links : firstly, because new varieties are very slowly 
formed, for variation is a very slow process, and 
natural selection can do nothing until favourable varia- 
tions chance to occur, and until a place in the natural 



polity of the country can be better filled by some 
modification of some one or more of its inhabitants. 
And such new places will depend on slow changes of 
climate, or on the occasional immigration of new inha- 
bitants, and, probably, in a still more important degree, 
on some of the old inhabitants becoming slowly modi- 
fied, with the new forms thus produced and the old 
ones acting and reacting on each other. So that, in 
any one region and at any one time, we ought only to 
see a few species presenting slight modifications of struc- 
ture in some degree permanent ; and this assuredly we 
do see. 

Secondly, areas now continuous must often have 
existed within the recent period in isolated portions, in 
which many forms, more especially amongst the classes 
which unite for each birth and wander much, may have 
separately been rendered sufficiently distinct to rank as 
representative species. In this case, intermediate vari- 
eties between the several representative species and 
their common parent, must formerly have existed in 
each broken portion of the land, but these links will 
have been supplanted and exterminated during the pro- 
cess of natural selection, so that they will no longer 
exist in a living state. 

Thirdly, when two or more varieties have been formed 
in different portions of a strictly continuous area, inter- 
mediate varieties will, it is probable, at first have been 
formed in the intermediate zones, but they will gene- 
rally have had a short duration. For these inter- 
mediate varieties will, from reasons already assigned 
(namely from what we know of the actual distribution 
of closely allied or representative species, and likewise 
of acknowledged varieties), exist in the intermediate 
zones in lesser numbers than the varieties which they 
tend to connect. From this cause alone the interme- 


diate varieties will be liable to accidental extermination ; 
and during the process of further modification through 
natural selection, they will almost certainly be beaten 
and supplanted by the forms which they connect; for 
these from existing in greater numbers will, in the 
aggregate, present more variation, and thus be further 
improved through natural selection and gain further 

Lastly, looking not to any one time, but to all time, 
if my theory be true, numberless intermediate varieties, 
linking most closely all the species of the same group 
together, must assuredly have existed ; but the very 
process of natural selection constantly tends, as has been 
so often remarked, to exterminate the parent-forms and 
the intermediate links. Consequently evidence of their 
former existence could be found only amongst fossil 
remains, which are preserved, as we shall in a future 
chapter attempt to show, in an extremely imperfect and 
intermittent record. 

On the origin and transitions of organic beings with 
peculiar habits and structure. — It has been asked by 
the opponents of such views as I hold, how, for instance, 
a land carnivorous animal could have been converted 
into one with aquatic habits ; for how could the animal 
in its transitional state have subsisted? It would be 
easy to show that within the same group carnivorous 
animals exist having every intermediate grade between 
truly aquatic and strictly terrestrial habits ; and as 
each exists by a struggle for life, it is clear that each is 
well adapted in its habits to its place in nature. Look 
at the Mustela vison of North America, which has 
webbed feet and which resembles an otter in its fur, 
short legs, and form of tail ; during summer this animal 
dives for and preys on fish, but during the long winter 


it leaves the frozen waters, and preys like other pole- 
cats on mice and land animals. If a different case had 
been taken, and it had been asked how an insectivorous 
quadruped could possibly have been converted into a 
flying bat, the question would have been far more diffi- 
cult, and I could have given no answer. Yet I think 
such difficulties have very little weight. 

Here, as on other occasions, I lie under a heavy dis- 
advantage, for out of the many striking cases which I 
have collected, I can give only one or two instances 
of transitional habits and structures in closely allied 
species of the same genus ; and of diversified habits, 
either constant or occasional, in the same species. And 
it seems to me that nothing less than a long list of such 
cases is sufficient to lessen the difficulty in any par- 
ticular case like that of the bat. 

Look at the family of squirrels ; here we have the 
finest gradation from animals with their tails only 
slightly flattened, and from others, as Sir J. Eichardson 
has remarked, with the posterior part of their bodies 
rather wide and with the skin on their flanks rather full, 
to the so-called flying squirrels ; and flying squirrels 
have their limbs and even the base of the tail united by 
a broad expanse of skin, which serves as a parachute 
and allows them to glide through the air to an asto- 
nishing distance from tree to tree. We cannot doubt 
that each structure is of use to each kind of squirrel in 
its own country, by enabling it to escape birds or beasts 
of prey, or to collect food more quickly, or, as there 
is reason to believe, by lessening the danger from occa- 
sional falls. But it does not follow from this fact that 
the structure of each squirrel is the best that it is pos- 
sible to conceive under all natural conditions. Let the 
climate and vegetation change, let other competing 
rodents or new beasts of prey immigrate, or old ones 


become modified, and all analogy would lead us to 
believe that some at least of the squirrels would decrease 
in numbers or become exterminated, unless they also 
became modified and improved in structure in a corre- 
sponding manner. Therefore, I can see no difficulty, 
more especially under changing conditions of life, in 
the continued preservation of individuals with fuller and 
fuller flank-membranes, each modification being useful, 
each being propagated, until by the accumulated effects 
of this process of natural selection, a perfect so-called 
flying squirrel was produced. 

Now look at the Galeopithecus or flying lemur, 
which formerly was falsely ranked amongst bats. It 
has an extremely wide flank-membrane, stretching from 
the corners of the jaw to the tail, and including the 
limbs and the elongated fingers: the flank-membrane 
is, also, furnished with an extensor muscle. Although 
no graduated links of structure, fitted for gliding through 
the air, now connect the Galeopithecus with the other 
Lemuridee, yet I can see no difficulty in supposing that 
such links formerly existed, and that each had been 
formed by the same steps as in the case of the less per- 
fectly gliding squirrels ; and that each grade of structure 
had been useful to its possessor. Nor can I see any in- 
superable difficulty in further believing it possible that 
the membrane-connected fingers and fore-arm of the 
Galeopithecus might be greatly lengthened by natural 
selection ; and this, as far as the organs of flight are 
concerned, would convert it into a bat. In bats which 
have the wing-membrane extended from the top of the 
shoulder to the tail, including the hind-legs, we per- 
haps see traces of an apparatus originally constructed 
for gliding through the air rather than for flight. 

If about a dozen genera of birds had become extinct 
or were unknown, who would have ventured to have 


surmised that birds might have existed which used their 
wings solely as flappers, like the logger-headed duck 
(Micropterus of Eyton) ; as fins in the water and front 
legs on the land, like the penguin ; as sails, like the 
ostrich ; and functionally for no purpose, like the 
Apteryx. Yet the structure of each of these birds is 
good for it, under the conditions of life to which it is 
exposed, for each has to live by a struggle ; but it is 
not necessarily the best possible under all possible con- 
ditions. It must not be inferred from these remarks 
that any of the grades of wing-structure here alluded to, 
which perhaps may all have resulted from disuse, indi- 
cate the natural steps by which birds have acquired 
their perfect power of flight ; but they serve, at least, 
to show what diversified means of transition are possible. 

Seeing that a few members of such water-breathing 
classes as the Crustacea and Mollusca are adapted to 
live on the land, and seeing that we have flying birds 
and mammals, flying insects of the most diversified 
types, and formerly had flying reptiles, it is conceivable 
that flying-fish, which now glide far through the air, 
slightly rising and turning by the aid of their fluttering 
fins, might have been modified into perfectly winged 
animals. If this had been effected, who would have 
ever imagined that in an early transitional state they 
had been inhabitants of the open ocean, and had used 
their incipient organs of flight exclusively, as far as we 
know, to escape being devoured by other fish ? 

When we see any structure highly perfected for any 
particular habit, as the wings of a bird for flight, we 
should bear in mind that animals displaying early tran- 
sitional grades of the structure will seldom continue to 
exist to the present day, for they will have been sup- 
planted by the very process of perfection through natural 
selection. Furthermore, we may conclude that transi- 


tional grades between structures fitted for very different 
habits of life will rarely have been developed at an early 
period in great numbers and under many subordinate 
forms. Thus, to return to our imaginary illustration of 
the flying-fish, it does not seem probable that fishes 
capable of true flight would have been developed under 
many subordinate forms, for taking prey of many kinds 
in many ways, on the land and in the water, until their 
organs of flight had come to a high stage of perfection, 
so as to have given them a decided advantage over 
other animals in the battle for life. Hence the chance 
of discovering species with transitional grades of struc- 
ture in a fossil condition will always be less, from their 
having existed in lesser numbers, than in the case of 
species with fully developed structures. 

I will now give two or three instances of diversified 
and of changed habits in the individuals of the same 
species. When either case occurs, it would be easy for 
natural selection to fit the animal, by some modification 
of its structure, for its changed habits, or exclusively 
for one of its several different habits. But it is difficult 
to tell, and immaterial for us, whether habits generally 
change first and structure afterwards ; or whether slight 
modifications of structure lead to changed habits ; both 
probably often change almost simultaneously. Of cases 
of changed habits it will suffice merely to allude to that 
of the many British insects which now feed on exotic 
plants, or exclusively on artificial substances. Of diver- 
sified habits innumerable instances could be given : I 
have often watched a tyrant flycatcher (Saurophagus 
sulphuratus) in South America, hovering over one spot 
and then proceeding to another, like a kestrel, and at 
other times standing stationary on the margin of water, 
and then dashing like a kingfisher at a fish. In our 
owp country the larger titmouse (Parus major) may be 


seen climbing branches, almost like a creeper ; it often, 
like a shrike, kills small birds by blows on the head ; 
and I have many times seen and heard it hammering 
the seeds of the yew on a branch, and thus breaking 
them like a nuthatch. In North America the black bear 
was seen by Hearne swimming for hours with widely 
open mouth, thus catching, like a whale, insects in the 
water. Even in so extreme a case as this, if the supply 
of insects were constant, and if better adapted compe- 
titors did not already exist in the country, I can see no 
difficulty in a race of bears being rendered, by natural 
selection, more and more aquatic in their structure 
and habits, with larger and larger mouths, till a creature 
was produced as monstrous as a whale. 

As we sometimes see individuals of a species following 
habits widely different from those both of their own 
species and of the other species of the same genus, we 
might expect, on my theory, that such individuals 
would occasionally have given rise to new species, having 
anomalous habits, and with their structure either slightly 
or considerably modified from that of their proper type. 
And such instances do occur in nature. Can a more 
striking instance of adaptation be given than that of a 
woodpecker for climbing trees and for seizing insects in 
the chinks of the bark ? Yet in North America there 
are woodpeckers which feed largely on fruit, and others 
with elongated wings which chase insects on the wing ; 
and on the plains of La Plata, where not a tree grows, 
there is a woodpecker, which in every essential part of its 
organisation, even in its colouring, in the harsh tone of 
its voice, and undulatory flight, told me plainly of its 
close blood-relationship to our common species ; yet it 
is a woodpecker which never climbs a tree ! 

Petrels are the most aerial and oceanic of birds, yet 
in the quiet Sounds of Tierra del Fuego, the Puffinuria 


berardi, in its general habits, in its astonishing power of 
diving, its manner of swimming, and of flying when 
unwillingly it takes flight, would be mistaken by any 
one for an auk or grebe ; nevertheless, it is essentially 
a petrel, but with many parts of its organisation pro- 
foundly modified. On the other hand, the acutest 
observer by examining the dead body of the water-ouzel 
would never have suspected its sub-aquatic habits ; yet 
this anomalous member of the strictly terrestrial thrush 
family wholly subsists by diving, — grasping the stones 
with its feet and using its wings under water. 

He who believes that each being has been created as 
we now see it, must occasionally have felt surprise when 
he has met with an animal having habits and structure 
not at all in agreement. What can be plainer than 
that the webbed feet of ducks and geese are formed 
for swimming ? yet there are upland geese with webbed 
feet which rarely or never go near the water ; and no 
one except Audubon has seen the frigate-bird, which 
has all its four toes webbed, alight on the surface of the 
sea. On the other hand, grebes and coots are emi- 
nently aquatic, although their toes are only bordered 
by membrane. What seems plainer than that the long 
toes of grallatores are formed for walking over swamps 
and floating plants, yet the water-hen is nearly as aquatic 
as the coot ; and the landrail nearly as terrestrial as the 
quail or partridge. In such cases, and many others 
could be given, habits have changed without a cor- 
responding change of structure. The webbed feet of the 
upland goose may be said to have become rudimentary 
in function, though not in structure. In the frigate- 
bird, the deeply-scooped membrane between the toes 
shows that structure has begun to change. 

He who believes in separate and innumerable acts of 
creation will say, that in these cases it has pleased the 


Creator to cause a being of one type to take the place 
of one of another type; but this seems to me only 
restating the fact in dignified language. He who be- 
lieves in the struggle for existence and in the principle 
of natural selection, will acknowledge that every organic 
being is constantly endeavouring to increase in numbers ; 
and that if any one being vary ever so little, either in 
habits or structure, and thus gain an advantage over 
some other inhabitant of the country, it will seize on 
the place of that inhabitant, however different it may 
be from its own place. Hence it will cause him no 
surprise that there should be geese and frigate-birds 
with webbed feet, either living on the dry land or most 
rarely alighting on the water ; that there should be 
long-toed corncrakes living in meadows instead of in 
swamps ; that there should be woodpeckers where not 
a tree grows ; that there should be diving thrushes, and 
petrels with the habits of auks. 

of extreme 'perfection and complication. — 
To suppose that the eye, with all its inimitable con- 
trivances for adjusting the focus to different distances, 
for admitting different amounts of light, and for the 
correction of spherical and chromatic aberration, could 
have been formed by natural selection, seems, I freely 
confess, absurd in the highest possible degree. Yet 
reason tells me, that if numerous gradations from a 
perfect and complex eye to one very imperfect and 
simple, each grade being useful to its possessor, can be 
shown to exist ; if further, the eye does vary ever so 
slightly, and the variations be inherited, which is cer- 
tainly the case ; and if any variation or modification 
in the organ be ever useful to an animal under changing 
conditions of life, then the difficulty of believing that 
a perfect and complex eye could be formed by natural 


selection, though insuperable by our imagination, can 
hardly be considered real. How a nerve comes to be 
sensitive to light, hardly concerns us more than how 
life itself first originated ; but I may remark that 
several facts make me suspect that any sensitive nerve 
may be rendered sensitive to light, and likewise to those 
coarser vibrations of the air which produce sound. 

In looking for the gradations by which an organ in 
any species has been perfected, we ought to look exclu- 
sively to its lineal ancestors ; but this is scarcely ever 
possible, and we are forced in each case to look to spe- 
cies of the same group, that is to the collateral descend- 
ants from the same original parent-form, in order to 
see what gradations are possible, and for the chance 
of some gradations having been transmitted from the 
earlier stages of descent, in an unaltered or little altered 
condition. Amongst existing Vertebrata, we find but a 
small amount of gradation in the structure of the eye, 
and from fossil species we can learn nothing on this 
head. In this great class we should probably have to 
descend far beneath the lowest known fossiliferous 
stratum to discover the earlier stages, by which the eye 
has been perfected. 

In the Articulata we can commence a series with an 
optic nerve merely coated with pigment, and without 
any other mechanism ; and from this low stage, nume- 
rous gradations of structure, branching off in two fun- 
damentally different lines, can be shown to exist, until 
we reach a moderately high stage of perfection. In 
certain crustaceans, for instance, there is a double 
cornea, the inner one divided into facets, within each 
of which there is a lens-shaped swelling. In other 
crustaceans the transparent cones which are coated by 
pigment, and which properly act only by excluding 
lateral pencils of light, are convex at their upper ends 


and must act by convergence ; and at their lower ends 
there seems to be an imperfect vitreous substance. With 
these facts, here far too briefly and imperfectly given, 
which show that there is much graduated diversity in 
the eyes of living crustaceans, and bearing in mind how 
small the number of living animals is in proportion to 
those which have become extinct, I can see no very 
great difficulty (not more than in the case of many 
other structures) in believing that natural selection 
has converted the simple apparatus of an optic nerve 
merely coated with pigment and invested by transparent 
membrane, into an optical instrument as perfect as is 
possessed by any member of the great Articulate class. 

He who will go thus far, if he find on finishing this 
treatise that large bodies of facts, otherwise inexpli- 
cable, can be explained by the theory of descent, ought 
not to hesitate to go further, and to admit that a struc- 
ture even as perfect as the eye of an eagle might be 
formed by natural selection, although in this case he 
does not know any of the transitional grades. His 
reason ought to conquer his imagination ; though I 
have felt the difficulty far too keenly to be surprised 
at any degree of hesitation in extending the principle 
of natural selection to such startling lengths. 

It is scarcely possible to avoid comparing the eye to 
a telescope. We know that this instrument has been 
perfected by the long-continued efforts of the highest 
' human intellects ; and we naturally infer that the eye 
has been formed by a somewhat analogous process. But 
i may not tins inference be presumptuous? Have we 
any right to assume that the Creator works by intel- 
lectual powers like those of man ? If we must compare 
the eye to an optical instrument, we ought in imagina- 
tion to take a thick layer of transparent tissue, with a 
nerve sensitive to light beneath, and then suppose every 


part of this layer to be continually changing slowly in 
density, so as to separate into layers of different densi- 
ties and thicknesses, placed at different distances from 
each other, and with the surfaces of each layer slowly 
changing in form. Further we must suppose that there 
is a power always intently watching each slight acci- 
dental alteration in the transparent layers ; and carefully 
selecting each alteration which, under varied circum- 
stances, may in any way, or in any degree, tend to pro- 
duce a distincter image. We must suppose each new 
state of the instrument to be multiplied by the million ; 
and each to be preserved till a better be produced, and 
then the old ones to be destroyed. In living bodies, 
variation will cause the slight alterations, generation 
will multiply them almost infinitely, and natural selec- 
tion will pick out with unerring skill each improvement. 
Let this process go on for millions on millions of years ; 
and during each year on millions of individuals of many 
kinds ; and may we not believe that a living optical 
instrument might thus be formed as superior to one of 
glass, as the works of the Creator are to those of man ? 

If it could be demonstrated that any complex organ 
existed, which could not possibly have been formed by 
numerous, successive, slight modifications, my theory 
would absolutely break down. But I can find out no 
such case. No doubt many organs exist of which we 
do not know the transitional grades, more especially if 
we look to much-isolated species, round which, accord- 
ing to my theory, there has been much extinction. Or 
again, if we look to an organ common to all the mem- 
bers of a large class, for in this latter case the organ 
must have been first formed at an extremely remote 
period, since which all the many members of the class 
have been developed ; and in order to discover the 
early transitional grades through which the organ has 


passed, we should have to look to very ancient ancestral 
forms, long since become extinct. 

We should be extremely cautious in concluding that 
an organ could not have been formed by transitional 
gradations of some kind. Numerous cases could be given 
amongst the lower animals of the same organ performing 
at the same time wholly distinct functions ; thus the ali- 
mentary canal respires, digests, and excretes in the larva 
of the dragon-fly and in the fish Cobites. In the Hydra, 
the animal may be turned inside out, and the exterior 
surface will then digest and the stomach respire. In 
such cases natural selection might easily specialise, if 
any advantage were thus gained, a part or organ, winch 
had performed two functions, for one function alone, 
and thus wholly change its nature by insensible steps. 
Two distinct organs sometimes perform simultaneously 
the same function in the same individual ; to give one 
instance, there are fish with gills or branchia? that 
breathe the ah- dissolved in the water, at the same time 
that they breathe free air in their swimbladders, this 
latter organ having a ductus pneumaticus for its supply, 
and being divided by highly vascular partitions. In 
these cases, one of the two organs might with ease be 
modified and perfected so as to perform all the work 
by itself, being aided during the process of modification 
by the other organ; and then this other organ might 
be modified for some other and quite distinct purpose, 
or be quite obliterated. 

The illustration of the swimbladder in fishes is a good 
one, because it shows us clearly the highly important fact 
that an organ originally constructed for one purpose, 
namely flotation, may be converted hito one for a wholly 
different purpose, namely respiration. The swimbladder 
has, also, been worked in as an accessory to the auditory 
organs of certain fish, or, for I do not know winch 


view is now generally held, a part of the auditory 
apparatus has been worked in as a complement to the 
swimbladcler. All physiologists admit that the swim- 
bladder is homologous, or " ideally similar," in position 
and structure with the lungs of the higher vertebrate 
animals : hence there seems to me to be no great diffi- 
culty in believing that natural selection has actually 
converted a swimbladder into a lung, or organ used ex- 
clusively for respiration. 

I can, indeed, hardly doubt that all vertebrate ani- 
mals having true lungs have descended by ordinary 
generation from an ancient prototype, of which we know 
nothing, furnished with a floating apparatus or swim- 
bladder. We can thus, as I infer from Professor Owen's 
interesting description of these parts, understand the 
strange fact that every particle of food and drink which 
we swallow has to pass over the orifice of the trachea, 
with some risk of falling into the lungs, notwithstanding 
the beautiful contrivance by which the glottis is closed. 
In the higher Vertebrata the branchiae have wholly dis- 
appeared — the slits on the sides of the neck and the 
loop-like course of the arteries still marking in the em- 
bryo their former position. But it is conceivable that 
the now utterly lost branchiae might have been gradually 
worked in by natural selection for some quite distinct 
purpose : in the same manner as, on the view entertained 
by some naturalists that the branchiae and dorsal scales 
of Annelids are homologous with the wings and wing- 
covers of insects, it is probable that organs which at a 
very ancient period served for respiration have been 
actually converted into organs of flight. 

In considering transitions of organs, it is so important 
to bear in mind the probability of conversion from one 
function to another, that I will give one more instance. 
Pedunculated cirripedes have two minute folds of skin, 


called by me the ovigerous frena, which serve, through 
the means of a sticky secretion, to retain the eggs until 
they are hatched within the sack. These cirripedes 
have no branchiae, the whole surface of the body and 
sack, including the small frena, serving for respiration. 
The Balanidae or sessile cirripedes, on the other hand, 
have no ovigerous frena, the eggs lying loose at the 
bottom of the sack, in the well-enclosed shell ; but they 
have large folded branchiae. Now I think no one will 
dispute that the ovigerous frena in the one family are 
strictly homologous with the branchiae of the other 
family ; indeed, they graduate into each other. There- 
fore I do not doubt that little folds of skin, winch ori- 
ginally served as ovigerous frena, but winch, likewise, 
very slightly aided the act of respiration, have been 
gradually converted by natural selection into branchiae, 
simply through an increase in their size and the oblite- 
ration of their adhesive glands. If all pedunculated 
cirripedes had become extinct, and they have already 
suffered far more extinction than have sessile cirripedes, 
who would ever have imagined that the branchiae in this 
latter family had originally existed as organs for pre- 
venting the ova from being washed out of the sack ? 

Although we must be extremely cautious in con- 
cluding that any organ could not possibly have been 
produced by successive transitional gradations, yet, un- 
doubtedly, grave cases of difficulty occur, some of which 
will be discussed in my future work. 

One of the gravest is that of neuter insects, which 
are often very differently constructed from either the 
males or fertile females ; but this case will be treated 
of in the next chapter. The electric organs of fishes 
offer another case of special difficulty ; it is impossible 
to conceive by what steps these wondrous organs have 
been produced ; but, as Owen and others have remarked, 


their intimate structure closely resembles that of common 
muscle ; and as it has lately been shown that Rays have 
an organ closely analogous to the electric apparatus, and 
yet do not, as Matteuchi asserts, discharge any electri- 
city, we must own that we are far too ignorant to argue 
that no transition of any kind is possible. 

The electric organs offer another and even more 
serious difficulty ; for they occur in only about a dozen 
fishes, of which several are widely remote in their 
affinities. Generally when the same organ appears in 
several members of the same class, especially if in 
members having very different habits of life, we may 
attribute its presence to inheritance from a common 
ancestor ; and its absence in some of the members to 
its loss through disuse or natural selection. But if the 
electric organs had been inherited from one ancient 
progenitor thus provided, we might have expected that 
all electric fishes would have been specially related to 
each other. Nor does geology at all lead to the belief 
that formerly most fishes had electric organs, which 
most of their modified descendants have lost. The 
presence of luminous organs in a few insects, belong- 
ing to different families and orders, offers a parallel 
case of difficulty. Other cases could be given ; for in- 
stance in plants, the very curious contrivance of a mass 
of pollen-grains, borne on a foot-stalk with a sticky 
gland at the end, is the same in Orchis and Asclepias, — 
genera almost as remote as possible amongst flowering 
plants. In all these cases of two very distinct spe- 
cies furnished with apparently the same anomalous 
organ, it should be observed that, although the general 
appearance and function of the organ may be the same, 
yet some fundamental difference can generally be de- 
tected. I am inclined to believe that in nearly the same 
way as two men have sometimes independently hit on 



the very same invention, so natural selection, working 
for the good of each being and taking advantage of 
analogous variations, has sometimes modified in very 
nearly the same manner two parts in two organic beings, 
which owe but little of their structure in common to 
inheritance from the same ancestor. 

Although in many cases it is most difficult to con- 
jecture by what transitions an organ could have arrived 
at its present state ; yet, considering that the proportion 
of living and known forms to the extinct and unknown 
is very small, I have been astonished how rarely an 
organ can be named, towards which no transitional 
grade is known to lead. The truth of this remark is 
indeed shown by that old canon in natural history of 
" Natura non facit saltum." We meet with this admis- 
sion in the writings of almost every experienced natu- 
ralist; or, as Milne Edwards has well expressed it, 
nature is prodigal in variety, but niggard in innovation. 
Why, on the theory of Creation, should this be so? 
Why should all the parts and organs of many inde- 
pendent beings, each supposed to have been separately 
created for its proper place in nature, be so invariably 
linked together by graduated steps ? Why should not 
Nature have taken a leap from structure to structure ? 
On the theory of natural selection, we can clearly 
understand why she should not ; for natural selection 
can act only by taking advantage of slight successive 
variations; she can never take a leap, but must ad- 
vance by the shortest and slowest steps. 

Organs of little apparent importance. — As natural 
selection acts by life and death, — by the preservation of 
individuals with any favourable variation, and by the 
destruction of those with any unfavourable deviation of 
structure, — I have sometimes felt much difficulty in 


understanding the origin of simple parts, of which the 
importance does not seem sufficient to cause the preser- 
vation of successively varying individuals. I have some- 
times felt as much difficulty, though of a very different 
kind, on this head, as in the case of an organ as perfect 
and complex as the eye. 

In the first place, we are much too ignorant in regard 
to the whole economy of any one organic being, to say 
what slight modifications would be of importance or 
not. In a former chapter I have given instances of 
most trifling characters, such as the down on fruit and 
the colour of the flesh, which, from determining the 
attacks of insects or from being correlated with con- 
stitutional differences, might assuredly be acted on by 
natural selection. The tail of the giraffe looks like an 
artificially constructed fly-flapper ; and it seems at first 
incredible that this could have been adapted for its 
present purpose by successive slight modifications, each 
better and better, for so trifling an object as driving 
away flies ; yet we should pause before being too posi- 
tive even in this case, for we know that the distribution 
and existence of cattle and other animals in South 
America absolutely depends on their power of resisting 
the attacks of insects : so that individuals which could 
by any means defend themselves from these small 
enemies, would be able to range into new pastures and 
thus gain a great advantage. It is not that the larger 
quadrupeds are actually destroyed (except in some rare 
cases) by the flies, but they are incessantly harassed and 
their strength reduced, so that they are more subject to 
disease, or not so well enabled in a coming dearth to 
search for food, or to escape from beasts of prey. 

Organs now of trifling importance have probably in 
some cases been of high importance to an early pro- 
genitor, and, after having been slowly perfected at a 



former period, have been transmitted in nearly the same 
state, although now become of very slight use ; and any 
actually injurious deviations in their structure will always 
have been checked by natural selection. Seeing how 
important an organ of locomotion the tail is in most 
aquatic animals, its general presence and use for many 
purposes in so many land animals, which in their lungs 
or modified swimbladders betray their aquatic origin, 
may perhaps be thus accounted for. A well-developed 
tail having been formed in an aquatic animal, it might 
subsequently come to be worked in for all sorts of pur- 
poses, as a fly-flapper, an organ of prehension, or as an 
aid in turning, as with the dog, though the aid must be 
slight, for the hare, with hardly any tail, can double 
quickly enough. 

In the second place, we may sometimes attribute 
importance to characters which are really of very little 
importance, and which have originated from quite 
secondary causes, independently of natural selection. 
We should remember that climate, food, &c, probably 
have some little direct influence on the organisation; 
that characters reappear from the law of reversion ; that 
correlation of growth will have had a most important 
influence in modifying various structures ; and finally, 
that sexual selection will often have largely modified 
the external characters of animals having a will, to 
give one male an advantage in fighting with another 
or in charming the females. Moreover when a modifi- 
cation of structure has primarily arisen from the above 
or other unknown causes, it may at first have been 
of no advantage to the species, but may subsequently 
have been taken advantage of by the descendants of 
the species under new conditions of life and with newly 
acquired habits. 

To give a few instances to illustrate these latter 


remarks. If green woodpeckers alone had existed, and 
we did not know that there were many black and pied 
kinds, I dare say that we should have thought that the 
green colour was a beautiful adaptation to hide this 
tree-frequenting bird from its enemies ; and conse- 
quently that it was a character of importance and might 
have been acquired through natural selection ; as it is, 
I have no doubt that the colour is due to some quite 
distinct cause, probably to sexual selection. A trailing 
bamboo in the Malay Archipelego climbs the loftiest 
trees by the aid of exquisitely constructed hooks clus- 
tered around the ends of the branches, and tins con- 
trivance, no doubt, is of the highest service to the 
plant; but as we see nearly similar hooks on many 
trees which are not climbers, the hooks on the bamboo 
may have arisen from unknown laws of growth, and 
have been subsequently taken advantage of by the 
plant undergoing further modification and becoming a 
climber. The naked skin on the head of a vulture is 
generally looked at as a direct adaptation for wallowing 
in putridity ; and so it may be, or it may possibly be 
due to the direct action of putrid matter ; but we 
should be very cautious in drawing any such inference, 
when we see that the skin on the head of the clean- 
feeding male turkey is likewise naked. The sutures in 
the skulls of young mammals have been advanced as a 
beautiful adaptation for aiding parturition, and no doubt 
they facilitate, or may be indispensable for this act; 
but as sutures occur in the skulls of young birds and 
reptiles, which have only to escape from a broken egg, 
we may infer that this structure has arisen from the 
laws of growth, and has been taken advantage of in the 
parturition of the higher animals. 

We are profoundly ignorant of the causes producing 
slight and unimportant variations ; and we are immedi- 


ately made conscious of this by reflecting on the differ- 
ences in the breeds of our domesticated animals in 
different countries, — more especially in the less civilized 
countries where there has been but little artificial selec- 
tion. Careful observers are convinced that a damp cli- 
mate affects the growth of the hair, and that with the 
hair the horns are correlated. Mountain breeds always 
differ from lowland breeds ; and a mountainous country 
would probably affect the hind limbs from exercising 
. them more, and possibly even the form of the pelvis ; 
and then by the law of homologous variation, the front 
limbs and even the head would probably be affected. 
The shape, also, of the pelvis might affect by pressure 
the shape of the head of the young in the womb. The 
laborious breathing necessary in high regions would, we 
have some reason to believe, increase the size of the 
chest ; and again correlation would come into play. 
Animals kept by savages in different countries 
often have to struggle for their own subsistence, 
and would be exposed to a certain extent to natural 
selection, and individuals with slightly different consti- 
tutions would succeed best under different climates ; 
and there is reason to believe that constitution and 
colour are correlated. A good observer, also, states that 
in cattle susceptibility to the attacks of flies is correlated 
with colour, as is the liability to be poisoned by certain 
plants ; so that colour would be thus subjected to the 
action of natural selection. But we are far too 
ignorant to speculate on the relative importance of 
the several known and unknown laws of valuation ; 
and I have here alluded to them only to show that, 
if we are unable to account for the characteristic 
differences of our domestic breeds, which nevertheless 
we generally admit to have arisen through ordinary 
generation, we ought not to lay too much stress on our 


ignorance of the precise cause of the slight analogous 
differences between species. I might have adduced for 
this same purpose the differences between the races of 
man, which are so strongly marked; I may add that 
some little light can apparently be thrown on the origin 
of these differences, chiefly through sexual selection of 
a particular kind, but without here entering on copious 
details my reasoning would appear frivolous. 

The foregoing remarks lead me to say a few words 
on the protest lately made by some naturalists, against 
the utilitarian doctrine that every detail of structure 
has been produced for the good of its possessor. They 
believe that very many structures have been created for 
beauty in the eyes of man, or for mere variety. This 
doctrine, if true, would be absolutely fatal to my theory. 
Yet I fully admit that many structures are of no direct 
use to their possessors. Physical conditions probably 
have had some little effect on structure, quite inde- 
pendently of any good thus gained. Correlation of 
growth has no doubt played a most important part, and 
a useful modification of one part will often have en- 
tailed on other parts diversified changes of no direct use. 
So again characters which formerly were useful, or 
which formerly had arisen from correlation of growth, 
or from other unknown cause, may reappear from the 
law of reversion, though now of no direct use. The 
effects of sexual selection, when displayed in beauty to 
charm the females, can be called useful only in rather 
a forced sense. But by far the most important con- 
sideration is that the chief part of the organisation of 
every being is simply due to inheritance; and conse- 
quently, though each being assuredly is well fitted for 
its place in nature, many structures now have no .direct 
relation to the habits of life of each species. Thus, we 
can hardly believe that the webbed feet of the upland 


goose or of the frigate-bird are of special use to these 
birds ; we cannot believe that the same bones in the 
arm of the monkey, in the fore leg of the horse, in the 
wing of the bat, and in the nipper of the seal, are of 
special use to these animals. We may safely attribute 
these structures to inheritance. But to the progenitor 
of the upland goose and of the frigate-bird, webbed feet 
no doubt were as useful as they now are to the most 
aquatic of existing birds. So we may believe that the 
progenitor of the seal had not a nipper, but a foot with 
five toes fitted for walking or grasping; and we may 
further venture to believe that the several bones in the 
limbs of the monkey, horse, and bat, which have been 
inherited from a common progenitor, were formerly of 
more special use to that progenitor, or its progenitors, 
than they now are to these animals having such widely 
diversified habits. Therefore we may infer that these 
several bones might have been acquired through 
natural selection, subjected formerly, as now, to the 
several laws of inheritance, reversion, correlation of 
growth, &c. Hence every detail of structure in every 
living creature (making some little allowance for the 
direct action of physical conditions) may be viewed, 
either as having been of special use to some ancestral 
form, or as being now of special use to the descendants 
of this form — either directly, or indirectly through the 
complex laws of growth. 

Natural selection cannot possibly produce any modifi- 
cation in any one species exclusively for the good of 
another species ; though throughout nature one species 
incessantly takes advantage of, and profits by, the struc- 
ture of another. But natural selection can and does 
often produce structures for the direct injury of other 
species, as we see in the fang of the adder, and in the 
ovipositor of the ichneumon, by which its eggs are depo- 


sited in the living bodies of other insects. If it could 
be proved that any part of the structure of any one 
species had been formed for the exclusive good of 
another species, it would annihilate my theory, for such 
could not have been produced through natural selec- 
tion. Although many statements may be found in 
works on natural history to this effect, I cannot find 
even one which seems to me of any weight. It is 
admitted that the rattlesnake has a poison-fang for its 
own defence and for the destruction of its prey; but 
some authors suppose that at the same time this snake 
is furnished with a rattle for its own injury, namely, to 
warn its prey to escape. I would almost as soon believe 
that the cat curls the end of its tail when preparing to 
spring, in order to warn the doomed mouse. But I have 
not space here to enter on this and other such cases. 

Natural selection will never produce in a being any- 
thing injurious to itself, for natural selection acts solely 
by and for the good of each. No organ will be formed, 
as Paley has remarked, for the purpose of causing pain 
or for doing an injury to its possessor. If a fair balance 
be struck between the good and evil caused by each 
part, each will be found on the whole advantageous. 
After the lapse of time, under changing conditions of 
life, if any part comes to be injurious, it will be modi- 
fied ; or if it be not so, the being will become extinct, 
as myriads have become extinct. 

Natural selection tends only to make each organic 
being as perfect as, or slightly more perfect than, the 
other inhabitants of the same country with which it has 
to struggle for existence. And we see that this is the 
degree of perfection attained under nature. The en- 
demic productions of New Zealand, for instance, are 
perfect one compared with another ; but they are now 
rapidly yielding before the advancing legions of plants 



and animals introduced from Europe. Natural selection 
will not produce absolute perfection, nor do we always 
meet, as far as we can judge, with this high standard 
under nature. The correction for the aberration of 
light is said, on high authority, not to be perfect even 
in that most perfect organ, the eye. If our reason 
leads us to admire with enthusiasm a multitude of 
inimitable contrivances in nature, this same reason tells 
us, though we may easily err on both sides, that some 
other contrivances are less perfect. Can we consider 
the sting of the wasp or of the bee as perfect, which, 
when used against many attacking animals, cannot be 
withdrawn, owing to the backward serratures, and so 
inevitably causes the death of the insect by tearing out 
its viscera ? 

If we look at the sting of the bee, as having origin- 
ally existed in a remote progenitor as a boring and 
serrated instrument, like that in so many members of 
the same great order, and which has been modified but 
not perfected for its present purpose, with the poison 
originally adapted to cause galls subsequently intensi- 
fied, we can perhaps understand how it is that the use 
of the sting should so often cause the insect's own 
death : for if on the whole the power of stinging be 
useful to the community, it will fulfil all the require- 
ments of natural selection, though it may cause the 
death of some few members. If we admire the truly 
wonderful power of scent by which the males of many 
insects find their females, can we admire the production 
for this single purpose of thousands of drones, which are 
utterly useless to the community for any other end, 
and which are ultimately slaughtered by their indus- 
trious and sterile sisters ? It may be difficult, but we 
ought to admire the savage instinctive hatred of the 
queen-bee, winch urges her instantly to destroy the 

Chap. VI. SUMMARY. 203 

young queens her daughters as soon as born, or to 
perish herself in the combat; for undoubtedly this is 
for the good of the community ; and maternal love or 
maternal hatred, though the latter fortunately is most 
rare, is all the same to the inexorable principle of 
natural selection. If we admire the several ingenious 
contrivances, by which the flowers of the orchis and of 
many other plants are fertilised through insect agency, 
can we consider as equally perfect the elaboration by 
our fir-trees of dense clouds of pollen, in order that a 
few granules may be wafted by a chance breeze on to 
the ovules ? 

Summary of Chapter. — We have in this chapter dis- 
cussed some of the difficulties and objections which may 
be urged against my theory. Many of them are very 
grave ; but I think that in the discussion light has been 
thrown on several facts, which on the theory of inde- 
pendent acts of creation are utterly obscure. We have 
seen that species at any one period are not indefinitely 
variable, and are not linked together by a multitude 
of intermediate gradations, partly because the process of 
natural selection will always be very slow, and will act, 
at any one time, only on a very few forms ; and partly 
because the very process of natural selection almost 
implies the continual supplanting and extinction of pre- 
ceding and intermediate gradations. Closely allied spe- 
cies, now living on a continuous area, must often have 
been formed when the area was not continuous, and 
when the conditions of life did not insensibly graduate 
away from one part to another. When two varieties 
are formed in two districts of a continuous area, an in- 
termediate variety will often be formed, fitted for an 
intermediate zone ; but from reasons assigned, the inter- 
mediate variety will usually exist in lesser numbers than 


the two forms which it connects ; consequently the two 
latter, during the course of further modification, from 
existing in greater numbers, will have a great advantage 
over the less numerous intermediate variety, and will 
thus generally succeed in supplanting and extermi- 
nating it. 

We have seen in this chapter how cautious we should 
be in concluding that the most different habits of life 
could not graduate into each other; that a bat, for 
instance, could not have been formed by natural selec- 
tion from an animal which at first could only glide 
through the air. 

We have seen that a species may under new condi- 
tions of life change its habits, or have diversified habits, 
with some habits very unlike those of its nearest con- 
geners. Hence we can understand, bearing in mind 
that each organic being is trying to live wherever it 
can live, how it has arisen that there are upland geese 
with webbed feet, ground woodpeckers, diving thrushes, 
and petrels with the habits of auks. 

Although the belief that an organ so perfect as the 
eye could have been formed by natural selection, is 
more than enough to stagger any one ; yet in the case 
of any organ, if we know of a long series of gradations 
in complexity, each good for its possessor, then, under 
changing conditions of life, there is no logical impossi- 
bility in the acquirement of any conceivable degree of 
perfection through natural selection. In the cases in 
which we know of no intermediate or transitional states, 
we should be very cautious in concluding that none 
could have existed, for the homologies of many organs 
and their intermediate states show that wonderful meta- 
morphoses in function are at least possible. For instance, 
a swim-bladder has apparently been converted into an 
air-breathing lung. The same organ having performed 

Chap. VI. SUMMAKY. 205 

simultaneously very different functions, and then having 
been specialised for one function ; and two very distinct 
organs having performed at the same time the same 
function, the one having been perfected whilst aided 
by the other, must often have largely facilitated 

We are far too ignorant, in almost every case, to be 
enabled to assert that any part or organ is so unim- 
portant for the welfare of a species, that modifications 
in its structure could not have been slowly accumulated 
by means of natural selection. But we may confidently 
believe that many modifications, wholly due to the laws 
of growth, and at first in no way advantageous to a spe- 
cies, have been subsequently taken advantage of by the 
still further modified descendants of this species. We 
may, also, believe that a part formerly of high import- 
ance has often been retained (as the tail of an aquatic 
animal by its terrestrial descendants), though it has 
become of such small importance that it could not, in 
its present state, have been acquired by natural selec- 
tion, — a power which acts solely by the preservation of 
profitable variations in the struggle for life. 

Natural selection will produce nothing in one species 
for the exclusive good or injury of another ; though it 
may well produce parts, organs, and excretions highly 
useful or even indispensable, or highly injurious to 
another species, but in all cases at the same time useful 
to the owner. Natural selection in each well-stocked 
country, must act chiefly through the competition of 
the inhabitants one with another, and consequently will 
produce perfection, or strength in the battle for life, only 
according to the standard of that country. Hence the 
inhabitants of one country, generally the smaller one, 
will often yield, as we see they do yield, to the inha- 
bitants of another and generally larger country. For in 


the larger country there will have existed more indi- 
viduals, and more diversified forms, and the competition 
will have been severer, and thus the standard of perfec- 
tion will have been rendered higher. Natural selection 
will not necessarily produce absolute perfection ; nor, as 
far as we can judge by our limited faculties, can absolute 
perfection be everywhere found. 

On the theory of natural selection we can clearly 
understand the full meaning of that old canon in natural 
history, " Natura non facit saltum." This canon, if 
we look only to the present inhabitants of the world, is 
not strictly correct, but if we include all those of past 
times, it must by my theory be strictly true. 

It is generally acknowledged that all organic beings 
have been formed on two great laws — Unity of Type, 
and the Conditions of Existence. By unity of type is 
meant that fundamental agreement in structure, which 
we see in organic beings of the same class, and which is 
' quite independent of their habits of life. On my theory, 
unity of type is explained by unity of descent. The 
expression of conditions of existence, so often insisted on 
by the illustrious Cuvier, is fully embraced by the prin- 
ciple of natural selection. For natural selection acts by 
either now adapting the varying parts of each being to 
its organic and inorganic conditions of life ; or by having 
adapted them during long-past periods of time : the 
adaptations being aided in some cases by use and dis- 
use, being slightly affected by the direct action of the 
external conditions of life, and being in all cases sub- 
jected to the several laws of growth. Hence, in fact, 
the law of the Conditions of Existence is the higher 
law ; as it includes, through the inheritance of former 
adaptations, that of Unity of Type. 


Chap. VII. INSTINCT. 207 



Instincts comparable with habits, but different in their origin — 
Instincts graduated — Aphides and ants — Instincts variable — 
Domestic instincts, their origin — Natural instincts of the cuckoo, 
ostrich, and parasitic bees — Slave-making ants — Hive-bee, its 
cell-making instinct — Difficulties on the theory of the Natural 
Selection of instincts — Neuter or sterile insects — Summary. 

The subject of instinct might have been worked into the 
previous chapters ; but I have thought that it would be 
more convenient to treat the subject separately, espe- 
cially as so wonderful an instinct as that of the hive- 
bee making its cells will probably have occurred to 
many readers, as a difficulty sufficient to overthrow my 
whole theory. I must premise, that I have nothing to 
do with the origin of the primary mental powers, any 
more than I have with that of life itself. We are con- 
cerned only with the diversities of instinct and of the 
other mental qualities of animals within the same class. 
I will not attempt any definition of instinct. It would 
be easy to show that several distinct mental actions are 
commonly embraced by this term ; but every one under- 
stands what is meant, when it is said that instinct impels 
the cuckoo to migrate and to lay her eggs in other birds' 
nests. An action, which we ourselves should require 
experience to enable us to perform, when performed by 
an animal, more especially by a very young one, without 
any experience, and when performed by many indivi- 
duals in the same way, without their knowing for what 
purpose it is performed, is usually said to be instinctive. 

208 * INSTINCT. Chap. VII. 

But I could show that none of these characters of 
instinct are universal. A little dose, as Pierre Huber 
expresses it, of judgment or reason, often conies into 
play, even in animals very low in the scale of nature. 

Frederick Cuvier and several of the older metaphy- 
sicians have compared instinct with habit. This com- 
parison gives, I think, a remarkably accurate notion of 
the frame of mind under which an instinctive action is 
performed, but not of its origin. How unconsciously 
many habitual actions are performed, indeed not rarely 
in direct opposition to our conscious will ! yet they may 
be modified by the will or reason. Habits easily become 
associated with other habits, and with certain periods of 
time and states of the body. When once acquired, they 
often remain constant throughout life. Several other 
points of resemblance between instincts and habits could 
be pointed out. As in repeating a well-known song, so 
in instincts, one action follows another by a sort of 
rhythm ; if a person be interrupted in a song, or in 
repeating anything by rote, he is generally forced to go 
back to recover the habitual train of thought: so P. 
Huber found it was with a caterpillar, winch makes a 
very complicated hammock ; for if he took a caterpillar 
which had completed its hammock up to, say, the sixth 
stage of construction, and put it into a hammock com- 
pleted up only to the third stage, the caterpillar simply 
re-performed the fourth, fifth, and sixth stages of con- 
struction. If, however, a caterpillar were taken out of 
a hammock made up, for instance, to the third stage, 
and were put into one finished up to the sixth stage, so 
that much of its work was already done for it, far from 
feeling the benefit of this, it was much embarrassed, and, 
in order to complete its hammock, seemed forced to 
start from the third stage, where it had left off, and thus 
tried to complete the already finished work. 


If we suppose any habitual action to become inhe- 
rited — and I think it can be shown that this does 
sometimes happen — then the resemblance between what 
originally was a habit and an instinct becomes so close 
as not to be distinguished. If Mozart, instead of playing 
the pianoforte at three years old with wonderfully little 
practice, had played a tune with no practice at all, he 
might truly be said to have done so instinctively. But 
it would be the most serious error to suppose that the 
greater number of instincts have been acquired by habit 
in one generation, and then transmitted by inheritance 
to succeeding generations. It can be clearly shown that 
the most wonderful instincts with which we are ac- 
quainted, namely, those of the hive-bee and of many 
ants, could not possibly have been thus acquired. 

It will be universally admitted that instincts are 
as important as corporeal structure for the welfare 
of each species, under its present conditions of life- 
Under changed conditions of life, it is at least possible 
that slight modifications of instinct might be profitable 
to a species ; and if it can be shown that instincts do 
vary ever so little, then I can see no difficulty in natural 
selection preserving and continually accumulating vari- 
ations of instinct to any extent that may be profitable. 
It is thus, as I believe, that all the most complex and 
wonderful instincts have originated. As modifications 
of corporeal structure arise from, and are increased by, 
use or habit, and are diminished or lost by disuse, so I do 
not doubt it has been with instincts. But I believe that 
the effects of habit are of quite subordinate importance 
to the effects of the natural selection of what may be 
called accidental variations of instincts ; — that is of vari- 
ations produced by the same unknown causes which pro- 
duce slight deviations of bodily structure. 

No complex instinct can possibly be produced through 

210 INSTINCT. Chap. VII. 

natural selection, except by the slow and gradual accu- 
mulation of numerous, slight, yet profitable, variations. 
Hence, as in the case of corporeal structures, we ought 
to find in nature, not the actual transitional gradations 
by which each complex instinct has been acquired — for 
these could be found only in the lineal ancestors of each 
species — but we ought to find in the collateral lines of 
descent some evidence of such gradations ; or we ought 
at least to be able to show that gradations of some kind 
are possible ; and this we certainly can do. I have 
been surprised to find, making allowance for the instincts 
of animals having been but little observed except in 
Europe and North America, and for no instinct being 
known amongst extinct species, how very generally gra- 
dations, leading to the most complex instincts, can be 
discovered. The canon of " Natura non facit saltum " 
applies with almost equal force to instincts as to bodily 
organs. Changes of instinct may sometimes be facili- 
tated by the same species having different instincts at 
different periods of life, or at different seasons of the 
year, or when placed under different circumstances, &c. ; 
in which case either one or the other instinct might 
be preserved by natural selection. And such instances 
of diversity of instinct in the same species can be shown 
to occur in nature. 

Again as in the case of corporeal structure, and con- 
formably with my theory, the instinct of each species is 
good for itself, but has never, as far as we can judge, 
been produced for the exclusive good of others. One of 
the strongest instances of an animal apparently per- 
forming an action for the sole good of another, with 
which I am acquainted, is that of aphides voluntarily 
yielding their sweet excretion to ants : that they do so 
voluntarily, the following facts show. I removed all the 
ants from a group of about a dozen aphides on a dock- 

Chap. VII. INSTINCT. 211 

plant, and prevented their attendance during several 
hours. After this interval, I felt sure that the aphides 
would want to excrete. I watched them for some time 
through a lens, but not one excreted ; I then tickled 
and stroked them with a hair in the same manner, as 
well as I could, as the ants do with their antennae ; but 
not one excreted. Afterwards I allowed an ant to visit 
them, and it immediately seemed, by its eager way of 
running about, to be well aware what a rich flock it had 
discovered ; it then began to play with its antennae on 
the abdomen first of one aphis and then of another ; 
and each aphis, as soon as it felt the antennae, imme- 
diately lifted up its abdomen and excreted a limpid 
drop of sweet juice, which was eagerly devoured by the 
ant. Even the quite young aphides behaved in this 
manner, showing that the action was instinctive, and 
not the result of experience. But as the excretion is 
extremely viscid, it is probably a convenience to the 
aphides to have it removed ; and therefore probably the 
aphides do not instinctively excrete for the sole good of 
the ants. Although I do not believe that any animal in 
the world performs an action for the exclusive good of 
another of a distinct species, yet each species tries to 
take advantage of the instincts of others, as each takes 
advantage of the weaker bodily structure of others. So 
again, in some few cases, certain instincts cannot be 
considered as absolutely perfect ; but as details on this 
and other such points are not indispensable, they may 
be here passed over. 

As some degree of variation in instincts under a 
state of nature, and the inheritance of such variations, 
are indispensable for the action of natural selection, as 
many instances as possible ought to have been here 
given ; but want of space prevents me. I can only 
assert, that instincts certainly do vary — for instance, 

212 INSTINCT. Chap. VII. 

the migratory instinct, both in extent and direction, 
and in its total loss. So it is with the nests of birds, 
which vary partly in dependence on the situations 
chosen, and on the nature and temperature of the 
country inhabited, but often from causes wholly un- 
known to us: Audubon has given several remarkable 
cases of differences in nests of the same species in the 
northern and southern United States. Fear of any 
particular enemy is certainly an instinctive quality, as 
may be seen in nestling birds, though it is strengthened 
by experience, and by the sight of fear of the same 
enemy in other animals. But fear of man is slowly 
acquired, as I have elsewhere shown, by various animals 
inhabiting desert islands ; and we may see an instance 
of this, even in England, in the greater wildness of 
all our large birds than of our small birds ; for the 
large birds have been most persecuted by man. We 
may safely attribute the greater wildness of our large 
birds to this cause ; for in uninhabited islands large 
birds are not more fearful than small ; and the magpie, 
so wary in England, is tame in Norway, as is the 
hooded crow in Egypt. 

That the general disposition of individuals of the same 
species, born in a state of nature, is extremely diversified, 
can be shown by a multitude of facts. Several cases 
also, could be given, of occasional and strange habits in 
certain species, which might, if advantageous to the 
species, give rise, through natural selection, to quite new 
instincts. But I am well aware that these general state- 
ments, without facts given in detail, can produce but a 
feeble effect on the reader's mind. I can only repeat my 
assurance, that I do not speak without good evidence. 

The possibility, or even probability, of inherited 
variations of instinct in a state of nature will be 
strengthened by briefly considering a few cases under 


domestication. We shall thus also be enabled to see 
the respective parts which habit and the selection of so- 
called accidental variations have played in modifying 
the mental qualities of our domestic animals. A number 
of curious and authentic instances could be given of 
the inheritance of all shades of disposition and tastes, 
and likewise of the oddest tricks, associated with certain 
frames of mind or periods of time. But let us look to 
the familiar case of the several breeds of dogs: it 
cannot be doubted that young pointers (I have myself 
seen a striking instance) will sometimes point and even 
back other dogs the very first time that they are taken 
out ; retrieving is certainly in some degree inherited by 
retrievers ; and a tendency to run round, instead of at, 
a flock of sheep, by shepherd-dogs. I cannot see that 
these actions, performed without experience by the 
young, and in nearly the same manner by each indi- 
vidual, performed with eager delight by each breed, and 
without the end being known, — for the young pointer 
can no more know that he points to aid his master, 
than the white butterfly knows why she lays her eggs 
on the leaf of the cabbage,— I cannot see that these 
actions differ essentially from true instincts. If we 
were to see one kind of wolf, when young and without 
any training, as soon as it scented its prey, stand motion- 
less like a statue, and then slowly crawl forward with a 
peculiar gait ; and another kind of wolf rushing round, 
instead of at, a herd of deer, and driving them to a 
distant point, we should assuredly call these actions in- 
stinctive. Domestic instincts, as they may be called, 
are certainly far less fixed or invariable than natural 
instincts ; but they have been acted on by far less rigor- 
ous selection, and have been transmitted for an incom- 
parably shorter period, under less fixed conditions of life. 
How strongly these domestic instincts, habits, and dis- 

214 INSTINCT. Chap. VII. 

positions are inherited, and how curiously they become 
mingled, is well shown when different breeds of dogs are 
crossed. Thus it is known that a cross with a bull-dog 
has affected for many generations the courage and obsti- 
nacy of greyhounds ; and a cross with a greyhound has 
given to a whole family of shepherd-dogs a tendency to 
hunt hares. These domestic instincts, when thus tested 
by crossing, resemble natural instincts, which in a like 
manner become curiously blended together, and for a 
long period exhibit traces of the instincts of either 
parent: for example, Le Koy describes a dog, whose 
great-grandfather was a wolf, and this dog showed a 
trace of its wild parentage only in one way, by not 
coming in a straight line to his master when called. 

Domestic instincts are sometimes spoken of as actions 
which have become inherited solely from long-continued 
and compulsory habit, but this, I think, is not true. 
No one would ever have thought of teaching, or pro- 
bably could have taught, the tumbler-pigeon to tumble, — 
an action which, as I have witnessed, is performed by 
young birds, that have never seen a pigeon tumble. 
We may believe that some one pigeon showed a slight ten- 
dency to this strange habit, and that the long-continued 
selection of the best individuals in successive generations 
made tumblers what they now are ; and near Glasgow 
there are house-tumblers, as I hear from Mr. Brent, 
which cannot fly eighteen inches high without going 
head over heels. It may be doubted whether any one 
would have thought of training a dog to point, had not 
some one dog naturally shown a tendency in this line ; 
and this is known occasionally to happen, as I once 
saw in a pure terrier. When the first tendency was 
once displayed, methodical selection and the inherited 
effects of compulsory training in each successive gene- 
ration would soon complete the work ; and unconscious 


selection is still at work, as each man tries to procure, 
without intending to improve the breed, dogs which will 
stand and hunt best. On the other hand, habit alone 
in some cases has sufficed ; no animal is more difficult 
to tame than the young of the wild rabbit; scarcely 
any animal is tamer than the young of the tame rabbit ; 
but I do not suppose that domestic rabbits have ever 
been selected for tameness ; and I presume that we 
must attribute the whole of the inherited change from 
extreme wildness to extreme tameness, simply to habit 
and long-continued close confinement. 

Natural instincts are lost under domestication : a re- 
markable instance of this is seen in those breeds of 
fowls which very rarely or never become "broody," 
that is, never wish to sit on their eggs. Familiarity 
alone prevents our seeing how universally and largely 
the minds of our domestic animals have been modified 
by domestication. It is scarcely possible to doubt that 
the love of man has become instinctive in the dog. All 
wolves, foxes, jackals, and species of the cat genus, when 
kept tame, are most eager to attack poultry, sheep, and 
pigs ; and this tendency has been found incurable in 
dogs which have been brought home as puppies from 
countries, such as Tierra del Fuego and Australia, where 
the savages do not keep these domestic animals. How 
rarely, on the other hand, do our civilised dogs, even 
when quite young, require to be taught not to attack 
poultry, sheep, and pigs! No doubt they occasionally 
do make an attack, and are then beaten ; and if not 
cured, they are destroyed; so that habit, with some 
degree of selection, has probably concurred in civilising 
by inheritance our dogs. On the other hand, young 
chickens have lost, wholly by habit, that fear of the dog 
and cat which no doubt was originally instinctive in 
them, in the same way as it is so plainly instinctive in 

216 INSTINCT. Chap. VII. 

young pheasants, though reared under a hen. It is not 
that chickens have lost all fear, but fear only of dogs 
and cats, for if the hen gives the danger-chuckle, they 
will run (more especially young turkeys) from under 
her, and conceal themselves in the surrounding grass or 
thickets ; and this is evidently done for the instinctive 
purpose of allowing, as we see in wild ground-birds, their 
mother to fly away. But this instinct retained by our 
chickens has become useless under domestication, for 
the mother-hen has almost lost by disuse the power of 

Hence, we may conclude, that domestic instincts have 
been acquired and natural instincts have been lost partly 
by habit, and partly by man selecting and accumulating 
during successive generations, peculiar mental habits 
and actions, which at first appeared from what we must 
in our ignorance call an accident. In some cases com- 
pulsory habit alone has sufficed to produce such inhe- 
rited mental changes ; in other cases compulsory habit 
has done nothing, and all has been the result of selec- 
tion, pursued both methodically and unconsciously ; but 
in most cases, probably, habit and selection have acted 

We shall, perhaps, best understand how instincts in a 
state of nature have become modified by selection, by 
considering a few cases. I will select only three, out of 
tne several which I shall have to discuss in my future 
work, — namely, the instinct which leads the cuckoo to 
lay her eggs in other birds' nests; the slave-making 
instinct of certain ants ; and the comb-making power of 
the hive-bee : these two latter instincts have generally, 
and most justly, been ranked by naturalists as the most 
wonderful of all known instincts. 

It is now commonly admitted that the more imme- 
diate and final cause of the cuckoo's instinct is, that 

Chap. VII. OF THE CUCKOO. 217 

she lays her eggs, not daily, but at intervals of two or 
three days ; so that, if she were to make her own nest 
and sit on her own eggs, those first laid would have 
to be left for some time unincubated, or there would be 
eggs and young birds of different ages in the same nest. 
If this were the case, the process of laying and hatching 
might be inconveniently long, more especially as she 
has to migrate at a very early period; and the first 
hatched young would probably have to be fed by the 
male alone. But the American cuckoo is in this pre- 
dicament; for she makes her own nest and has eggs 
and young successively hatched, all at the same time. 
It has been asserted that the American cuckoo occa- 
sionally lays her eggs in other birds' nests ; but I hear 
on the high authority of Dr. Brewer, that this is a mis- 
take. Nevertheless, I could give several instances of 
various birds which have been known occasionally to 
lay their eggs in other birds' nests. Now let us sup- 
pose that the ancient progenitor of our European 
cuckoo had the habits of the American cuckoo ; but 
that occasionally she laid an egg in another bird's 
nest. If the old bird profited by this occasional habit, 
or if the young were made more vigorous by advantage 
having been taken of the mistaken maternal instinct of 
another bird, than by their own mother's care, encum- 
bered as she can hardly fail to be by having eggs and 
young of different ages at the same time ; then the old 
birds or the fostered young would gain an advantage. 
And analogy would lead me to believe, that the young 
thus reared would be apt to follow by inheritance the 
occasional and aberrant habit of their mother, and in 
their turn would be apt to lay their eggs in other birds' 
nests, and thus be successful in rearing their young. 
By a continued process of this nature, I believe that the 
strange instinct of our cuckoo could be, and has been, 


218 INSTINCT. Chap. VII. 

generated. I may add that, according to Dr. Gray 
and to some other observers, the European cuckoo has 
not utterly lost all maternal love and care for her own 

The occasional habit of birds laying their eggs in 
other birds' nests, either of the same or of a distinct 
species, is not very uncommon with the Gallinaceae ; 
and this perhaps explains the origin of a singular 
instinct in the allied group of ostriches. For several 
hen ostriches, at least in the case of the American 
species, unite and lay first a few eggs in one nest and 
then in another ; and these are hatched by the males. 
This instinct may probably be accounted for by the fact 
of the hens laying a large number of eggs ; but, as in 
the case of the cuckoo, at intervals of two or three days. 
This instinct, however, of the American ostrich has not 
as yet been perfected ; for a surprising number of eggs 
lie strewed over the plains, so that in one day's hunting 
I picked up no less than twenty lost and wasted eggs. 

Many bees are parasitic, and always lay their eggs in 
the nests of bees of other kinds. This case is more re- 
markable than that of the cuckoo ; for these bees have 
not only their instincts but their structure modified in 
accordance with their parasitic habits ; for they do not 
possess the pollen-collecting apparatus which would be 
necessary if they had to store food for their own young. 
Some species, likewise, of Sphegidae (wasp-like insects) 
are parasitic on other species ; and M. Fabre has lately 
shown good reason for believing that although the 
Tachytes nigra generally makes its own burrow and 
stores it with paralysed prey for its own larvae to feed 
on, yet that when this insect finds a burrow already 
made and stored by another sphex, it takes advantage 
of the prize, and becomes for the occasion parasitic. In 
this case, as with the supposed case of the cuckoo, I can 


see no difficulty in natural selection making an occa- 
sional habit permanent, if of advantage to the species, 
and if the insect whose nest and stored food are thus 
feloniously appropriated, be not thus exterminated. 

Slave-making instinct. — This remarkable instinct was 
first discovered in the Formica (Polyerges) rufescens 
by Pierre Huber, a better observer even than his cele- 
brated father. This ant is absolutely dependent on its 
slaves ; without their aid, the species would certainly 
become extinct in a single year. The males and fertile 
females do no work. The workers or sterile females, 
though most energetic and courageous in capturing 
slaves, do no other work. They are incapable ofr 
making their own nests, or of feeding their own 
larvae. When the old nest is found inconvenient, and 
they have to migrate, it is the slaves which determine 
the migration, and actually carry their masters in their 
jaws. So utterly helpless are the masters, that when 
Huber shut up thirty of them without a slave, but with 
plenty of the food which they like best, and with their 
larva? and pupae to stimulate them to Avork, they did 
nothing; they could not even feed themselves, and 
many perished of hunger. Huber then introduced a 
single slave (F. fusca), and she instantly set to work, 
fed and saved the survivors ; made some cells and 
tended the larvae, and put all to rights. What can be 
more extraordinary than these well-ascertained facts? 
If we had not known of any other slave-making ant, it 
would have been hopeless to have speculated how so 
wonderful an instinct could have been perfected. 

Formica sanguinea was likewise first discovered by 
P. Huber to be a slave-making ant. This species is 
found in the southern parts of England, and its habits 
have been attended to by Mr. F. Smith, of the British 


220 INSTINCT. Chap. VII. 

Museum, to whom I am much indebted for information 
on this and other subjects. Although fully trusting to 
the statements of Huber and Mr. Smith, I tried to 
approach the subject in a sceptical frame of mind, as 
any one may well be excused for doubting the truth of 
so extraordinary and oflious an instinct as that of 
making slaves. Hence I will give the observations which 
I have myself made, in some little detail. I opened 
fourteen nests of F. sanguinea, and found a few slaves in 
all. Males and fertile females of the slave-species are 
found only in their own proper communities, and have 
never been observed in the nests of F. sanguinea. The 
slaves are black and not above half the size of their red 
masters, so that the contrast in their appearance is very 
great. When the nest is slightly disturbed, the slaves 
occasionally come out, and like their masters are much 
agitated and defend the nest : when the nest is much 
disturbed and the larvae and pupae are exposed, the 
slaves work energetically with their masters in carrying 
them away to a place of safety. Hence, it is clear, 
that the slaves feel quite at home. During the months 
of June and July, on three successive years, I have 
watched for many hours several nests in Surrey and 
Sussex, and never saw a slave either leave or enter 
a nest. As, during these months, the slaves are very 
few in number, I thought that they might behave differ- 
ently when more numerous ; but Mr. Smith informs me 
that he has watched the nests at various hours during 
May, June and August, both in Surrey and Hampshire, 
and has never seen the slaves, though present in large 
numbers in August, either leave or enter the nest. 
Hence he considers them as strictly household slaves. 
The masters, on the other hand, may be constantly 
seen bringing in materials for the nest, and food of all 
lands. During the present year, however, in the month 


of July, I came across a community with an unusually 
large stock of slaves, and I observed a few slaves mingled 
with their masters leaving the nest, and marching along 
the same road to a tall Scotch -fir-tree, twenty-five yards 
distant, which they ascended together, probably in search 
of aphides or cocci. According to Huber, who had ample 
opportunities for observation, in Switzerland the slaves 
habitually work with their masters in making the nest, 
and they alone open and close the doors in the morning 
and evening ; and, as Huber expressly states, their 
principal office is to search for aphides. This differ- 
ence in the usual habits of the masters and slaves 
in the two countries, probably depends merely on the 
slaves being captured in greater numbers in Switzerland 
than in England. 

One day I fortunately chanced to witness a migration 
from one nest to another, and it was a most interesting 
spectacle to behold the masters carefully carrying, as 
Huber has described, their slaves in their jaws. Another 
day my attention was struck by about a score of the 
slave-makers haunting the same spot, and evidently not 
in search of food ; they approached and were vigorously 
repulsed by an independent community of the slave 
species (F. fusca) ; sometimes as many as three of these 
ants clinging to the legs of the slave-making F. san- 
guinea. The latter ruthlessly killed their small oppo- 
nents, and carried their dead bodies as food to their 
nest, twenty-nine yards distant; but they were pre- 
vented from getting any pupae to rear as slaves. I 
then dug up a small parcel of the pupae of F. fusca 
from another nest, and put them down on a bare spot 
near the place of combat; they were eagerly seized, 
and carried off by the tyrants, who perhaps fancied 
that, after all, they had been victorious in their late 

222 INSTINCT. Chap. VII. 

At the same time I laid on the same place a small 
parcel of the pupae of another species, F. flava, with a 
few of these little yellow ants still clinging to the frag- 
ments of the nest. This species is sometimes, though 
rarely, made into slaves, as has been described by Mr. 
Smith. Although so small a species, it is very cour- 
ageous, and I have seen it ferociously attack other ants. 
In one instance I found to my surprise an independent 
community of F. flava under a stone beneath a 
nest of the slave-making F. sanguinea ; and when I 
had accidentally disturbed both nests, the little ants 
attacked their big neighbours with surprising coinage. 
Now I was curious to ascertain whether F. sanguinea 
could distinguish the pupae of F. fusca, which they 
habitually make into slaves, from those of the little and 
furious F. flava, which they rarely capture, and it was 
evident that they did at once distinguish them : for we 
have seen that they eagerly and instantly seized the 
pupae of F. fusca, whereas they were much terrified 
when they came across the pupae, or even the earth 
from the nest of F. flava, and quickly ran away ; but in 
about a quarter of an hour, shortly after all the little 
yellow ants had crawled away, they took heart and car- 
ried off the pupae. 

One evening I visited another community of F. san- 
guinea, and found a number of these ants entering their 
nest, carrying the dead bodies of F. fusca (showing that 
it was not a migration) and numerous pupae. I traced 
the returning file burthened with booty, for about forty 
yards, to a very thick clump of heath, whence I saw the 
last individual of F. sanguinea emerge, carrying a pupa ; 
but I was not able to find the desolated nest in the 
thick heath. The nest, however, must have been close 
at hand, for two or three individuals of F. fusca were 
rushing about in the greatest agitation, and one was 


perched motionless with its own pupa in its mouth on 
the top of a spray of heath over its ravaged home. 

Such are the facts, though they did not need confirma- 
tion by me, in regard to the wonderful instinct of 
making slaves. Let it be observed what a contrast the 
instinctive habits of F. sanguinea present with those of 
the F. rufescens. The latter does not build its own nest, 
does not determine its own migrations, does not collect 
food for itself or its young, and cannot even feed 
itself : it is absolutely dependent on its numerous slaves. 
Formica sanguinea, on the other hand, possesses much 
fewer slaves, and in the early part of the summer ex- 
tremely few. The masters determine when and where 
a new nest shall be formed, and when they migrate, the 
masters carry the slaves. Both in Switzerland and 
England the slaves seem to have the exclusive care of 
the larvae, and the masters alone go on slave-making 
expeditions. In Switzerland the slaves and masters 
work together, making and bringing materials for the 
nest : both, but chiefly the slaves, tend, and milk as it 
may be called, their aphides ; and thus both collect food 
for the community. In England the masters alone 
usually leave the nest to collect building materials and 
food for themselves, their slaves and larva?. So that the 
masters in this country receive much less service from 
their slaves than they do in Switzerland. 

By what steps the instinct of F. sanguinea originated 
I will not pretend to conjecture. But as ants, which are 
not slave-makers, will, as I have seen, carry off pupa? of 
other species, if scattered near their nests, it is possible 
that pupa? originally stored as food might become de- 
veloped ; and the ants thus unintentionally reared would 
then follow their proper instincts, and do what work 
they could. If their presence proved useful to the 
species which had seized them — if it were more advan- 

224 INSTINCT. Chap. VII. 

tageous to this species to capture workers than to pro- 
create them — the habit of collecting pupse originally for 
food might by natural selection be strengthened and 
rendered permanent for the very different purpose of 
raising slaves. When the instinct was once acquired, 
if carried out to a much less extent even than in our 
British F. sanguinea, which, as we have seen, is less 
aided by its slaves than the same species in Switzerland, 
I can see no difficulty in natural selection increasing and 
modifying the instinct — always supposing each modifi- 
cation to be of use to the species — until an ant was 
formed as abjectly dependent on its slaves as is the 
Formica rufescens. 

Cell-making instinct of the Hive-Bee. — I will not here 
enter on minute details on this subject, but will merely 
give an outline of the conclusions at which I have arrived. 
He must be a dull man who can examine the exquisite' 
structure of a comb, so beautifully adapted to its end, 
without enthusiastic admiration. We hear from mathe- 
maticians that bees have practically solved a recondite 
problem, and have made their cells of the proper shape 
to hold the greatest possible amount of honey, with the 
least possible consumption of precious wax in their con- 
struction. It has been remarked that a skilful work- 
man, with fitting tools and measures, would find it very 
difficult to make cells of wax of the true form, though 
this is perfectly effected by a crowd of bees working in 
a dark hive. Grant whatever instincts you please, and it 
seems at first quite inconceivable how they can make all 
the necessary angles and planes, or even perceive when 
they are correctly made. But the difficulty is not 
nearly so great as it at first appears: all this beautiful 
work can be shown, I think, to follow from a few very 
simple instincts. 


I was led to investigate this subject by Mr. Water- 
house, who has shown that the form of the cell stands 
in close relation to the presence of adjoining cells ; and 
the following view may, perhaps, be considered only as 
a modification of his theory. Let us look to the great 
principle of gradation, and see whether Nature does not 
reveal to us her method of work. At one end of a short 
series we have humble-bees, which use their old cocoons 
to hold honey, sometimes adding to them short tubes of 
wax, and likewise making separate and very irregular 
rounded cells of wax. At the other end of the series we 
have the cells of the hive-bee, placed in a double layer : 
each cell, as is well known, is an hexagonal prism, with 
the basal edges of its six sides bevelled so as to join on 
to a pyramid, formed of three rhombs. These rhombs 
have certain angles, and the three which form the pyra- 
midal base of a single cell on one side of the comb, enter 
into the composition of the bases of three adjoining cells 
on the opposite side. In the series between the extreme 
perfection of the cells of the hive-bee and the simplicity 
of those of the humble-bee, we have the cells of the 
Mexican Melipona domestica, carefully described and 
figured by Pierre Huber. The Melipona itself is inter- 
mediate in structure between the hive and humble bee, 
but more nearly related to the latter : it forms a nearly 
regular waxen comb of cylindrical cells, in winch the 
young are hatched, and, in addition, some large cells of 
wax for holding honey. These latter cells are nearly 
spherical and of nearly equal sizes, and are aggregated 
into an irregular mass. But the important point to 
notice, is that these cells are always made at that 
degree of nearness to each other, that they would have 
intersected or broken into each other, if the spheres had 
been completed ; but this is never permitted, the bees 
building perfectly flat walls of wax between the spheres 


226 INSTINCT. Chap. VII. 

which thus tend to intersect. Hence each cell consists 
of an outer spherical portion and of two, three, or 
more perfectly fiat surfaces, according as the cell ad- 
joins two, three, or more other cells. When one cell 
comes into contact with three other cells, which, from 
the spheres being nearly of the same size, is very 
frequently and necessarily the case, the three flat sur- 
faces are united into a pyramid ; and this pyramid, as 
Huber has remarked, is manifestly a gross imitation of 
the three-sided pyramidal basis of the cell of the hive- 
bee. As in the cells of the hive-bee, so here, the three 
plane surfaces in any one cell necessarily enter into the 
construction of three adjoining cells. It is obvious that 
the Melipona saves wax by this manner of building ; for 
the flat walls between the adjoining cells are not double, 
but are of the same thickness as the outer spherical 
portions, and yet each flat portion forms a part of two 

Reflecting on this case, it occurred to me that if the 
Melipona had made its spheres at some given distance 
from each other, and had made them of equal sizes and 
had arranged them symmetrically in a double layer, the 
resulting structure would probably have been as perfect 
as the comb of the hive-bee. Accordingly I wrote to 
Professor Miller, of Cambridge, and this geometer has 
kindly read over the folloAving statement, drawn up from 
his information, and tells me that it is strictly correct : — 

If a number of equal spheres be described with their 
centres placed in two parallel layers ; with the centre 
of each sphere at the distance of radius x V 2, or 
radius X 141421 (or at some lesser distance), from the 
centres of the six surrounding spheres in the same 
layer ; and at the same distance from the centres of the 
adjoining spheres in the other and parallel layer ; then, 
if planes of intersection between the several spheres in 


both layers be formed, there will result a double layer of 
hexagoual prisms united together by pyramidal bases 
formed of three rhombs ; and the rhombs and the sides 
of the hexagonal prisms will have every angle identi- 
cally the same with the best measurements which have 
been made of the cells of the hive-bee. 

Hence we may safely conclude that if we could 
slightly modify the instincts already possessed by the 
Melipona, and in themselves not very wonderful, this 
bee would make a structure as wonderfully perfect as 
that of the hive-bee. We must suppose the Melipona 
to make her cells truly spherical, and of equal sizes ; 
and this would not be very surprising, seeing that she 
already does so to a certain extent, and seeing what 
perfectly cylindrical burrows in wood many insects can 
make, apparently by turning round on a fixed point. 
We must suppose the Melipona to arrange her cells in 
level layers, as she already does her cylindrical cells ; 
and we must further suppose, and this is the greatest 
difficulty, that she can somehow judge accurately at 
what distance to stand from her fellow-labourers when 
several are making their spheres ; but she is already 
so far enabled to judge of distance, that she always 
describes her spheres so as to intersect largely ; and 
then she unites the points of intersection by perfectly 
Hat surfaces. AVe have further to suppose, but this is 
no difficulty, that after hexagonal prisms have been 
formed by the intersection of adjoining spheres in the 
same layer, she can prolong the hexagon to any length 
requisite to hold the stock of honey ; in the same 
way as the rude humble-bee adds cylinders of wax 
to the circular mouths of her old cocoons. By such 
modifications of instincts in themselves not very won- 
derful, — hardly more wonderful than those which guide 
a bird to make its nest, — I believe that the hive-bee 

228 INSTINCT. Chap. VII. 

has acquired, through natural selection, her inimitable 
architectural powers. 

But this theory can be tested by experiment. Follow- 
ing the example of Mr. Tegetmeier, I separated two 
combs, and put between them a long, thick, square strip 
of wax: the bees instantly began to excavate minute 
circular pits in it; and as they deepened these little 
pits, they made them wider and wider until they were 
converted into shallow basins, appearing to the eye per- 
fectly time or parts of a sphere, and of about the dia- 
meter of a cell. It was most interesting to me to ob- 
serve that wherever several bees had begun to excavate 
these basins near together, they had begun their work 
at such a distance from each other, that by the time the 
basins had acquired the above stated width (*. e. about 
the width of an ordinary cell), and were in depth about 
one sixth of the diameter of the sphere of which they 
formed a part, the rims of the basins intersected or 
broke into each other. As soon as this occurred, the 
bees ceased to excavate, and began to build up flat walls 
of wax on the lines of intersection between the basins, 
so that each hexagonal prism was built upon the fes- 
tooned edge of a smooth basin, instead of on the straight 
edges of a three-sided pyramid as in the case of ordinary 

I then put into the hive, instead of a thick, square 
piece of wax, a thin and narrow, knife-edged ridge, 
coloured with vermilion. The bees instantly began on 
both sides to excavate little basins near to each other, in 
the same way as before ; but the ridge of wax was so 
thin, that the bottoms of the basins, if they had been 
excavated to the same depth as in the former experi- 
ment, would have broken into each other from the 
opposite sides. The bees, however, did not suffer this 
to happen, and they stopped their excavations in due 


time ; so that the basins, as soon as they had been a 
little deepened, came to have flat bottoms ; and these flat 
bottoms, formed by thin little plates of the vermilion 
wax having been left nngnawed, were situated, as far 
as the eye could judge, exactly along the planes of 
imaginary intersection between the basins on the op- 
posite sides of the ridge of wax. In parts, only little 
bits, in other parts, large portions of a rhombic plate 
had been left between the opposed basins, but the work, 
from the unnatural state of things, had not been neatly 
performed. The bees must have worked at very nearly 
the same rate on the opposite sides of the ridge of ver- 
milion wax, as they circularly gnawed away and deep- 
ened the basins on both sides, in order to have succeeded 
in thus leaving flat plates between the basins, by 
stopping work along the intermediate planes or planes 
of intersection. 

Considering how flexible thin wax is, I do not see 
that there is any difficulty in the bees, whilst at work on 
the two sides of a strip of wax, perceiving when they 
have gnawed the wax away to the proper thinness, 
and then stopping their work. In ordinary combs it 
has appeared to me that the bees do not always succeed 
in working at exactly the same rate from the opposite 
sides ; for I have noticed half-completed rhombs at the 
base of a just-commenced cell, which were slightly con- 
cave on one side, where I suppose that the bees had ex- 
cavated too quickly, and convex on the opposed side, 
where the bees had worked less quickly. In one well- 
marked instance, I put the comb back into the hive, and 
allowed the bees to go on working for a short time, and 
again examined the cell, and I found that the rhombic 
plate had been completed, and had become perfectly flat : 
it was absolutely impossible, from the extreme thinness 
of the little rhombic plate, that they could have effected 

230 INSTINCT. Chap. VII. 

this by gnawing away the convex side ; and I suspect 
that the bees in such cases stand in the opposed cells 
and push and bend the ductile and warm wax (which 
as I have tried is easily done) into its proper interme- 
diate plane, and thus flatten it. 

From the experiment of the ridge of vermilion wax, 
we can clearly see that if the bees were to build for 
themselves a thin wall of wax, they could make their 
cells of the proper shape, by standing at the proper dis- 
tance from each other, by excavating at the same rate, 
and by endeavouring to make equal spherical hollows, 
but never allowing the spheres to break into each 
other. Now bees, as may be clearly seen by examining 
the edge of a growing comb, do make a rough, circum- 
ferential wall or rim all round the comb ; and they 
gnaw into this from the opposite sides, always working 
circularly as they deepen each cell. They do not make 
the whole three-sided pyramidal base of any one cell at 
the same time, but only the one rhombic plate which 
stands on the extreme growing margin, or the two plates, 
as the case may be ; and they never complete the upper 
edges of the rhombic plates, until the hexagonal walla 
are commenced. Some of these statements differ from 
those made by the justly celebrated elder Huber, 
but I am convinced of their accuracy ; and if I had 
space, I could show that they are conformable with my 

Huber's statement that the veiy first cell is excavated 
out of a little parallel-sided wall of wax, is not, as far as 
I have seen, strictly correct ; the first commencement 
having always been a little hood of wax ; but I will 
not here enter on these details. We see how important 
a part excavation plays in the construction of the cells ; 
but it would be a great error to suppose that the bees 
cannot build up a rough wall of wax in the proper 


position — that is, along the plane of intersection between 
two adjoining spheres. I have several specimens show- 
ing clearly that they can do this. Even in the rude 
circumferential rim or wall of wax round a growing 
comb, flexures may sometimes be observed, correspond- 
ing in position to the planes of the rhombic basal plates 
of future cells. But the rough wall of wax has in every 
case to be finished off, by being largely gnawed away 
on both sides. The manner in which the bees build is 
curious ; they always make the first rough wall from ten 
to twenty times thicker than the excessively thin finished 
wall of the cell, which will ultimately be left. We shall 
understand how they work, by supposing masons first to 
pile up a broad ridge of cement, and then to begin cutting 
it away equally on both sides near the ground, till a 
smooth, very thin wall is left in the middle ; the masons 
always piling up the cut-away cement, and adding fresh 
cement, on the summit of the ridge. We shall thus 
have a thin wall steadily growing upward ; but always 
crowned by a gigantic coping. From all the cells, 
both those just commenced and those completed, being 
thus crowned by a strong coping of wax, the bees can 
cluster and crawl over the comb without injuring the 
delicate hexagonal walls, winch are only about one four- 
hundredth of an inch in thickness ; the plates of the 
pyramidal basis being about twice as thick. By this sin- 
gular manner of building, strength is continually given 
to the comb, with the utmost ultimate economy of wax. 
It seems at first to add to the difficulty of understand- 
ing how the cells are made, that a multitude of bees all 
work together ; one bee after working a short time at 
one cell going to another, so that, as Huber has stated, 
a score of individuals work even at the commencement 
of the first cell. I was able practically to show this 
fact, by covering the edges of the hexagonal walls 

232 INSTINCT. Chap. VII. 

of a single cell, or the extreme margin of the circumfer- 
ential rim of a growing comb, with an extremely thin 
layer of melted vermilion wax ; and I invariably found 
that the colour was most delicately diffused by the bees 
— as delicately as a painter could have done with his 
brush — by atoms of the coloured wax having been taken 
from the spot on which it had been placed, and worked 
into the growing edges of the cells all round. The work 
of construction seems to be a sort of balance struck 
between many bees, all instinctively standing at the 
same relative distance from each other, all trying to 
sweep equal spheres, and then building up, or leaving 
ungnawed, the planes of intersection between these 
spheres. It was really curious to note in cases of diffi- 
culty, as when two pieces of comb met at an angle, how 
often the bees would entirely pull down and rebuild in 
different ways the same cell, sometimes recurring to a 
shape which they had at first rejected. 

When bees have a place on winch they can stand in 
their proper positions for working, — for instance, on a 
slip of wood, placed directly under the middle of a comb 
growing downwards so that the comb has to be built over 
one face of the slip — in this case the bees can lay the 
foundations of one wall of a new hexagon, in its strictly 
proper place, projecting beyond the other completed 
cells. It suffices that the bees should be enabled to 
stand at their proper relative distances from each other 
and from the walls of the last completed cells, and then, 
by striking imaginary spheres, they can build up a wall 
intermediate between two adjoining spheres; but, as far 
as I have seen, they never gnaw away and finish off the 
angles of a cell till a large part both of that cell and of 
the adjoining cells has been built. This capacity in 
bees of laying down under certain circumstances a 
rough wall in its proper place between two just-com- 


menced cells, is important, as it bears on a fact, 
which seems at first quite subversive of the foregoing 
theory ; namely, that the cells on the extreme margin of 
wasp-combs are sometimes strictly hexagonal ; but I 
have not space here to enter on this subject. Nor does 
there seem to me any great difficulty in a single insect 
(as in the case of a queen- wasp) making hexagonal cells, 
if she work alternately on the inside and outside of two 
or three cells commenced at the same time, always 
standing at the proper relative distance from the parts 
of the cells just begun, sweeping spheres or cylinders, 
and building up intermediate planes. It is even conceiv- 
able that an insect might, by fixing on a point at which 
to commence a cell, and then moving outside, first to 
one point, and then to five other points, at the proper 
relative distances from the central point and from each 
other, strike the planes of intersection, and so make an 
isolated hexagon : but I am not aware that any such 
case has been observed ; nor would any good be derived 
from a single hexagon being built, as in its construction 
more materials would be required than for a cylinder. 

As natural selection acts only by the accumulation of 
slight modifications of structure or instinct, each profit- 
able to the individual under its conditions of life, it may 
reasonably be asked, how a long and graduated succession 
of modified architectural instincts, all tending towards the 
present perfect plan of construction, could have profited 
the progenitors of the hive-bee ? I think the answer is 
not difficult : it is known that bees are often hard pressed 
to get sufficient nectar ; and I am informed by Mr. 
Tegetmeier that it has been experimentally found that 
no less than from twelve to fifteen pounds of dry sugar 
are consumed by a hive of bees for the secretion of each 
pound of wax ; so that a prodigious quantity of fluid nectar 
must be collected and consumed by the bees in a hive for 

234 INSTINCT. Chap. VII. 

the secretion of the wax necessary for the construction of 
their combs. Moreover, many bees have to remain idle 
for many days during the process of secretion. A large 
store of honey is indispensable to support a large stock 
of bees during the winter ; and the security of the hiye 
is known mainly to depend on a large number of bees 
being supported. Hence the saving of wax by largely 
saving honey must be a most important element of suc- 
cess in any family of bees. Of course the success of any 
species of bee may be dependent on the number of its 
parasites or other enemies, or on quite distinct causes, 
and so be altogether independent of the quantity of 
honey which the bees could collect. But let us suppose 
that this latter circumstance determined, as it probably 
often does determine, the numbers of a humble-bee which 
could exist in a country ; and let us further suppose 
that the community lived throughout the winter, and con- 
sequently required a store of honey : there can in tins 
case be no doubt that it would be an advantage to our 
humble-bee, if a slight modification of her instinct led 
her to make her waxen cells near together, so as to 
intersect a little; for a wall in common even to two 
adjoining cells, would save some little wax. Hence it 
would continually be more and more advantageous to 
our humble-bee, if she were to make her cells more and 
more regular, nearer together, and aggregated into a 
mass, like the cells of the Melipona; for in this case 
a large part of the bounding surface of each cell would 
serve to bound other cells, and much wax would be 
saved. Again, from the same cause, it would be 
advantageous to the Melipona, if she were to make her 
cells closer together, and more regular in every way 
than at present ; for then, as we have seen, the 
spherical surfaces would wholly disappear, and would 
all be replaced by plane surfaces; and the Melipona 


would make a comb as perfect as that of the hive-bee. 
Beyond this stage of perfection in architecture, natural 
selection could not lead; for the comb of the hive- 
bee, as far as we can see, is absolutely perfect in eco- 
nomising wax. 

Thus, as I believe, the most wonderful of all known 
instincts, that of the hive-bee, can be explained by 
natural selection having taken advantage of numerous, 
successive, slight modifications of simpler instincts; 
natural selection having by slow degrees, more and 
more perfectly, led the bees to sweep equal spheres 
at a given distance from each other in a double 
layer, and to build up and excavate the wax along 
the planes of intersection. The bees, of course, no 
more knowing that they swept their spheres at one 
particular distance from each other, than they know 
what are the several angles of the hexagonal prisms 
and of the basal rhombic plates. The motive power 
of the process of natural selection having been economy 
of wax ; that individual swarm which wasted least 
honey in the secretion of wax, having succeeded best, 
and having transmitted by inheritance its newly acquired 
economical instinct to new swarms, which in their turn 
will have had the best chance of succeeding in the 
struggle for existence. 

No doubt many instincts of very difficult explanation 
could be opposed to the theory of natural selection, 
— cases, in which we cannot see how an instinct could 
possibly have originated; cases, in which no interme- 
diate gradations are known to exist; cases of instinct 
of apparently such trifling importance, that they could 
hardly have been acted on by natural selection ; cases of 
instincts almost identically the same in animals so re- 
mote in the scale of nature, that we cannot account 

236 INSTINCT. Chap. VII. 

for their similarity by inheritance from a common 
parent, and must therefore believe that they have 
been acquired by independent acts of natural selection. 
I will not here enter on these several cases, but will 
confine myself to one special difficulty, which at first 
appeared to me insuperable, and actually fatal to my 
whole theory. I allude to the neuters or sterile females 
in insect-communities : for these neuters often differ 
widely in instinct and in structure from both the males 
and fertile females, and yet, from being sterile, they 
cannot propagate their kind. 

The subject well deserves to be discussed at great 
length, but I will here take only a single case, that 
of working or sterile ants. How the workers have 
been rendered sterile is a difficulty ; but not much 
greater than that of any other striking modification of 
structure ; for it can be shown that some insects and 
other articulate animals in a state of nature occasionally 
become sterile ; and if such insects had been social, and 
it had been profitable to the community that a number 
should have been annually born capable of work, but in- 
capable of procreation, I can see no very great difficulty 
in this being effected by natural selection. But I must 
pass over this preliminary difficulty. The great difficulty 
lies in the working ants differing widely from both the 
males and the fertile females in structure, as in the shape 
of the thorax and in being destitute of wings and some- 
times of eyes, and in instinct. As far as instinct alone 
is concerned, the prodigious difference in this respect 
between the workers and the perfect females, would 
have been far better exemplified by the hive-bee. If a 
working ant or other neuter insect had been an animal 
in the ordinary state, I should have unhesitatingly 
assumed that all its characters had been slowly acquired 
through natural selection; namely, by an individual 


having been born with some slight profitable modifi- 
cation of structure, this being inherited by its offspring, 
which again varied and were again selected, and so 
onwards. But with the working ant we have an insect 
differing greatly from its parents, yet absolutely sterile ; 
so that it could never have transmitted successively 
acquired modifications of structure or instinct to its pro- 
geny. It may well be asked how is it possible to recon- 
cile this case with the theory of natural selection ? 

First, let it be remembered that we have innumerable 
instances, both in our domestic productions and in those 
in a state of nature, of all sorts of differences of struc- 
ture which have become correlated to certain ages, and 
to either sex. We have differences correlated not only 
to one sex, but to that short period alone when the re- 
productive system is active, as in the nuptial plumage of 
many birds, and in the hooked jaws of the male salmon. 
We have even slight differences in the horns of different 
breeds of cattle in relation to an artificially imperfect 
state of the male sex ; for oxen of certain breeds have 
longer horns than in other breeds, in comparison with 
the horns of the bulls or cows of these same breeds. 
Hence I can see no real difficulty in any character 
having become correlated with the sterile condition of 
certain members of insect-communities: the difficulty 
lies in understanding how such correlated modifications 
of structure could have been slowly accumulated by 
natural selection. 

This difficulty, though appearing insuperable, is 
lessened, or, as I believe, disappears, when it is re- 
membered that selection may be applied to the family, 
as well as to the individual, and may thus gain the 
desired end. Thus, a well-flavoured vegetable is cooked, 
and the individual is destroyed ; but the horticulturist 
sows seeds of the same stock, and confidently expects to 

238 INSTINCT. Chap. VII. 

get nearly the same variety ; breeders of cattle 'wish the 
flesh and fat to be well marbled together ; the animal 
has been slaughtered, but the breeder goes with confi- 
dence to the same family. I have such faith in the 
powers of selection, that I do not doubt that a breed 
of cattle, always yielding oxen with extraordinarily long 
horns, could be slowly formed by carefully watching 
which individual bulls and cows, when matched, produced 
oxen with the longest horns ; and yet no one ox could ever 
have propagated its kind. Thus I believe it has been 
with social insects : a slight modification of structure, or 
instinct, correlated with the sterile condition of certain 
members of the community, has been advantageous to 
the community : consequently the fertile males and 
females of the same community flourished, and trans- 
mitted to their fertile offspring a tendency to produce 
sterile members having the same modification. And 
I believe that this process has been repeated, until that 
prodigious amount of difference between the fertile and 
sterile females of the same species has been produced, 
which we see in many social insects. 

But we have not as yet touched on the climax of the 
difficulty; namely, the fact that the neuters of several 
ants differ, not only from the fertile females and males, 
but from each other, sometimes to an almost incredible 
degree, and are thus divided into two or even three 
castes. The castes, moreover, do not generally gra- 
duate into each other, but are perfectly well defined ; 
being as distinct from each other, as are any two species 
of the same genus, or rather as any two genera of the 
same family. Thus in Eciton, there are working and 
soldier neuters, with jaws and instincts extraordinarily 
different: in Cryptocerus, the workers of one caste alone 
carry a wonderful sort of shield on their heads, the use 
of winch is quite unknown: in the Mexican Blyrme- 


cocystus, the workers of one caste never leave the nest ; 
they are fed by the workers of another caste, and they 
have an enormously developed abdomen which secretes 
a sort of honey, supplying the place of that excreted by 
the aphides, or the domestic cattle as they may be called, 
which our European ants guard or imprison. 

It will indeed be thought that I have an overweening 
confidence in the principle of natural selection, when I 
do not admit that such wonderful and well-established 
facts at once annihilate my theory. In the simpler 
case of neuter insects all of one caste or of the same 
kind, which have been rendered by natural selection, as 
I believe to be quite possible, different from the fertile 
males and females, — in this case, we may safely conclude 
from the analogy of ordinary variations, that each suc- 
cessive, slight, profitable modification did not probably 
at first appear in all the individual neuters in the same 
nest, but in a few alone ; and that by the long-continued 
selection of the fertile parents winch produced most 
neuters with the profitable modification, all the neuters 
ultimately came to have the desired character. On this 
view we ought occasionally to find neuter-insects of the 
same species, in the same nest, presenting gradations of 
structure ; and this we do find, even often, considering how 
few neuter-insects out of Europe have been carefully 
examined. Mr. F. Smith has shown how surprisingly the 
neuters of several British ants differ from each other in 
size and sometimes in colour; and that the extreme 
forms can sometimes be perfectly linked together by in- 
dividuals taken out of the same nest : I have myself 
compared perfect gradations of this kind. It often 
happens' that the larger or the smaller sized workers 
are the most numerous ; or that both large and small 
are numerous, with those of an intermediate size 
scanty in numbers. Formica flava has larger and 

240 INSTINCT. Chap. VII. 

smaller workers, with some of intermediate size ; and, 
in this species, as Mr. F. Smith has observed, the 
larger workers have simple eyes (ocelli), which though 
small can be plainly distinguished, whereas the smaller 
workers have their ocelli rudimentary. Having care- 
fully dissected several specimens of these workers, I 
can affirm that the eyes are far more rudimentary in 
the smaller workers than can be accounted for merely 
by their proportionally lesser size ; and I fully believe, 
though I dare not assert so positively, that the workers 
of intermediate size have their ocelli in an exactly inter- 
mediate condition. So that we here have two bodies 
of sterile workers in the same nest, differing not only in 
size, but in their organs of vision, yet connected by some 
few members in an intermediate condition. I may 
digress by adding, that if the smaller workers had been 
the most useful to the community, and those males and 
females had been continually selected, winch produced 
more and more of the smaller workers, until all the 
workers had come to be in this condition ; we should 
then have had a species of ant with neuters very 
nearly in the same condition with those of Myrmica. 
For the workers of Myrmica have not even rudiments 
of ocelli, though the male and female ants of this genus 
have well-developed ocelli. 

I may give one other case : so confidently did I 
expect to find gradations in important points of structure 
between the different castes of neuters in the same spe- 
cies, that I gladly availed myself of Mr. F. Smith's offer 
of numerous specimens from the same nest of the driver 
ant (Anomma) of West Africa. The reader will per- 
haps best appreciate the amount of difference in these 
workers, by my giving not the actual measurements, 
but a strictly accurate illustration: the difference was 
the same as if we were to see a set of workmen building 


a house of whom many were five feet four inches high, 
and many sixteen feet high ; but we must suppose that 
the larger workmen had heads four instead of three 
times as big as those of the smaller men, and jaws nearly 
five times as big. The jaws, moreover, of the working 
ants of the several sizes differed wonderfully in shape, 
and in the form and number of the teeth. But the 
important fact for us is, that though the workers can be 
grouped into castes of different sizes, yet they graduate 
insensibly into each other, as does the widely-different 
structure of their jaws. I speak confidently on this 
latter point, as Mr. Lubbock made drawings for me 
with the camera lucida of the jaws which I had dis- 
sected from the workers of the several sizes. 

With these facts before me, I believe that natural 
selection, by acting on the fertile parents, could form 
a species which should regularly produce neuters, 
either all of large size with one form of jaw, or all of 
small size with jaws having a widely different structure ; 
or lastly, and this is our climax of difficulty, one set of 
workers of one size and structure, and simultaneously 
another set of workers of a different size and structure ; 
— a graduated series having been first formed, as in 
the case of the driver ant, and then the extreme forms, 
from being the most useful to the community, having 
been produced in greater and greater numbers through 
the natural selection 'of the parents which generated 
them ; until none with an intermediate structure were 

Thus, as I believe, the wonderful fact of two distinctly 
defined castes of sterile workers existing in the same 
nest, both widely different from each other and from 
their parents, has originated. We can see how useful 
their production may have been to a social community 
of insects, on the same principle that the division of 


242 INSTINCT. Chap. VII. 

labour is useful to civilised man. As ants work by 
inherited instincts and by inherited tools or weapons, 
and not by acquired knowledge and manufactured instru- 
ments, a perfect division of labour could be effected 
with them only by the workers being sterile ; for had 
they been fertile, they would have intercrossed, and 
their instincts and structure would have become blended. 
And nature has, as I believe, effected this admirable 
division of labour in the communities of ants, by the 
means of natural selection. But I am bound to confess, 
that, with all my faith in this principle, I should never 
have anticipated that natural selection could have been 
efficient in so high a degree, had not the case of these 
neuter insects convinced me of the fact. I have, there- 
fore, discussed this case, at some little but wholly insuf- 
ficient length, in order to show the power of natural 
selection, and likewise because tins is by far the most 
serious special difficulty, which my theory has encoun- 
tered. The case, also, is very interesting, as it proves 
that with animals, as with plants, any amount of modi- 
fication in structure can be effected by the accumula- 
tion of numerous, slight, and as we must call them acci- 
dental, variations, which are in any manner profitable, 
without exercise or habit having come into play. For 
no amount of exercise, or habit, or volition, in the utterly 
sterile members of a community could possibly have af- 
fected the structure or instincts of the fertile members, 
which alone leave descendants. I am surprised that no 
one has advanced this demonstrative case of neuter in- 
sects, against the well-known doctrine of Lamarck. 

Summary. — I have endeavoured briefly in this chapter 
to show that the mental qualities of our domestic 
animals vary, and that the variations are inherited. 
Still more briefly I have attempted to show that in- 

Chap. VII. SUMMAKY. 243 

stincts vary slightly in a state of nature. No one will 
dispute that instincts are of the highest importance to 
each animal. Therefore I can see no difficulty, under 
changing conditions of life, in natural selection accumu- 
lating slight modifications of instinct to any extent, 
in any useful direction. In some cases habit or use 
and disuse have probably come into play. I do not 
pretend that the facts given in this chapter strengthen 
in any great degree my theory ; but none of the cases 
of difficulty, to the best of my judgment, annihilate it. 
On the other hand, the fact that instincts are not always 
absolutely perfect and are liable to mistakes ; — that no 
instinct has been produced for the exclusive good of 
other animals, but that each animal takes advantage of 
the instincts of others ; — that the canon in natural his- 
tory, of " natura non facit saltum" is applicable to in- 
stincts as well as to corporeal structure, and is plainly 
explicable on the foregoing views, but is otherwise inex- 
plicable, — all tend to corroborate the theory of natural 

This theory is, also, strengthened by some few other 
facts in regard to instincts ; as by that common case of 
closely allied, but certainly distinct, species, when in- 
habiting distant parts of the world and living under 
considerably different conditions of life, yet often retain- 
ing nearly the same instincts. For instance, we can 
understand on the principle of inheritance, how it is 
that the thrush of South America lines its nest with 
mud, in the same peculiar manner as does our British 
thrush : how it is that the male wrens (Troglodytes) of 
North America, build " cock-nests," to roost in, like the 
males of our distinct Kitty- wrens, — a habit wholly unlike 
that of any other known bird. Finally, it may not be 
a logical deduction, but to my imagination it is far 
more satisfactory to look at such instincts as the young 

M 2 

244 INSTINCT. Chap. VII. 

cuckoo ejecting its foster-brothers, — ants making slaves, 
— the larvae of ichneumonidae feeding within the live 
bodies of caterpillars, — not as specially endowed or 
created instincts, but as small consequences of one 
general law, leading to the advancement of all organic 
beings,; namely, multiply, vary, let the strongest live 
and the weakest die. 

Chai>. VIII. HYBRIDISM. 245 



Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close inter- 
breeding, removed by domestication — Laws governing the sterility 
of hybrids — Sterility not a special endowment, but incidental 
on other differences — Causes of the sterility of first crosses and 
of hybrids — Parallelism between the effects of changed con- 
ditions of life and crossing — Fertility of varieties when crossed 
and of their mongrel offspring not universal — Hybrids and 
mongrels compared independently of their fertility — Summary. 

The view generally entertained by naturalists is that 
species, when intercrossed, have been specially endowed 
with the quality of sterility, in order to prevent the 
confusion of all organic forms. This view certainly 
seems at first probable, for species within the same 
country could hardly have kept distinct had they been, 
capable of crossing freely. The importance of the fact 
that hybrids are very generally sterile, has, I think, 
been much underrated by some late writers. On the 
theory of natural selection the case is especially im- 
portant, inasmuch as the sterility of hybrids could not 
possibly be of any advantage to them, and therefore 
could not have been acquired by the continued preser- 
vation of successive profitable degrees of sterility. I 
hope, however, to be able to show that sterility is not 
a specially acquired or endowed quality, but is inci- 
dental on other acquired differences. 

In treating this subject, two classes of facts, to a 
large extent fundamentally different, have generally 
been confounded together ; namely, the sterility of two 

246 HYBEIDISM. Chap. VI11. 

species when first crossed, and the sterility of the hybrids 
produced from them. 

Pure species have of course their organs of reproduc- 
tion in a perfect condition, yet when intercrossed they 
produce either few or no offspring. Hybrids, on the 
other hand, have their reproductive organs functionally 
impotent, as may be clearly seen in the state of the 
male element in both plants and animals ; though the 
organs themselves are perfect in structure, as far as the 
microscope reveals. In the first case the two sexual 
elements which go to form the embryo are perfect ; in 
the second case they are either not at all developed, or 
are imperfectly developed. This distinction is import- 
ant, when the cause of the sterility, which is common to 
the two cases, has to be considered. The distinction has 
probably been slurred over, owing to the sterility in 
both cases being looked on as a special endowment, 
beyond the province of our reasoning powers. 

The fertility of varieties, that is of the forms known 
or believed to have descended from common parents, 
when intercrossed, and likewise the fertility of their 
mongrel offspring, is, on my theory, of equal im- 
portance with the sterility of species; for it seems to 
make a broad and clear distinction between varieties 
and species. 

First, for the sterility of species when crossed and of 
their hybrid offspring. It is impossible to study the 
several memoirs and works of those two conscien- 
tious and admirable observers, Kolreuter and Gartner, 
who almost devoted their lives to this subject, without 
being deeply impressed with the high generality of 
some degree of sterility. Kolreuter makes the ride 
universal ; but then he cuts the knot, for in ten cases 
in which he found two forms, considered by most 
authors as distinct species, quite fertile together, he 


unhesitatingly ranks them as varieties. Gartner, also, 
makes the rule equally universal ; and he disputes the 
entire fertility of Kolreuter's ten cases. But in these 
and in many other cases, Gartner is obliged carefully 
to count the seeds, in order to show that there is 
any degree of sterility. He always compares the 
maximum number of seeds produced by two species 
when crossed and by their hybrid offspring, with the 
average number produced by both pure parent-species 
in a state of nature. But a serious cause of error seems 
to me to be here introduced : a plant to be hybridised 
must be castrated, and, what is often more important, 
must be secluded in order to prevent pollen being 
brought to it by insects from other plants. Nearly all the 
plants experimentised on by Gartner were potted, and 
apparently were kept in a chamber in his house. That 
these processes are often injurious to the fertility of a 
plant cannot be doubted ; for Gartner gives in his table 
about a score of cases of plants which he castrated, and 
artificially fertilised with their own pollen, and (exclud- 
ing all cases such as the Leguminosse, in which there 
is an acknowledged difficulty in the manipulation) half 
of these twenty plants had their fertility in some degree 
impaired. Moreover, as Gartner during several years 
repeatedly crossed the primrose and cowslip, which we 
have such good reason to believe to be varieties, 
and only once or twice succeeded in getting fertile 
seed; as he found the common red and blue pim- 
pernels (Anagallis arvensis and ccerulea), which the 
best botanists rank as varieties, absolutely sterile to- 
gether; and as he came to the same concluson in 
several other analogous cases ; it seems to me that we 
may well be permitted to doubt whether many other 
species are really so sterile, when intercrossed, as Gart- 
ner believes. 



It is certain, on the one hand, that the sterility of 
various species when crossed is so different in degree 
and graduates away so insensibly, and, on the other 
hand, that the fertility of pure species is so easily 
affected by various circumstances, that for all practical 
purposes it is most difficult to say where perfect fertility 
ends and sterility begins. I think no better evidence 
of this can be required than that the two most experi- 
enced observers who have ever lived, namely, Kolreuter 
and Gartner, should have arrived at diametrically oppo- 
site conclusions in regard to the very same species. 
It is also most instructive to compare — but I have not 
space here to enter on details — the evidence advanced 
by our best botanists on the question whether certain 
doubtful forms should be ranked as species or varieties, 
with the evidence from fertility adduced by different 
hybridisers, or by the same author, from experiments 
made during different years. It can thus be shown that 
neither sterility nor fertility affords any clear distinction 
between species and varieties; but that the evidence 
from this source graduates away, and is doubtful in the 
same degree as is the evidence derived from other con- 
stitutional and structural differences. 

In regard to the sterility of hybrids in successive 
generations ; though Gartner was enabled to rear some 
hybrids, carefully guarding them from a cross with 
either pure parent, for six or seven, and in one case for 
ten generations, yet he asserts positively that their fer- 
tility never increased, but generally greatly decreased. 
I do not doubt that this is usually the case, and that 
the fertility often suddenly decreases in the first few 
generations. Nevertheless I believe that in all these 
experiments the fertility has been diminished by an 
independent cause, namely, from close interbreeding. 
I have collected so large a body of facts, showing 


that close interbreeding lessens fertility, and, on the 
other hand, that an occasional cross with a distinct in- 
dividual or variety increases fertility, that I cannot doubt 
the correctness of this almost universal belief amongst 
breeders. Hybrids are seldom raised by experimen- 
talists in great numbers ; and as the parent-species, or 
other allied hybrids, generally grow in the same garden, 
the visits of insects must be carefully prevented during 
the flowering season : hence hybrids will generally be 
fertilised during each generation by their own indi- 
vidual pollen ; and I am con viced that this would be 
injurious to their fertility, already lessened by their 
hybrid origin. I am strengthened in this conviction 
by a remarkable statement repeatedly made by 
Gartner, namely, that if even the less fertile hybrids 
be artificially fertilised with hybrid pollen of the same 
kind, their fertility, notwithstanding the frequent ill 
effects of manipulation, sometimes decidedly increases, 
and goes on increasing. Now, in artificial fertilisation 
pollen is as often taken by chance (as I know from my 
own experience) from the anthers of another flower, as 
from the anthers of the flower itself which is to be 
fertilised ; so that a cross between two flowers, though 
probably on the same plant, would be thus effected. 
Moreover, whenever complicated experiments are in 
progress, so careful an observer as Gartner would have 
castrated his hybrids, and this would have insured in 
each generation a cross with the pollen from a distinct 
flower, either from the same plant or from another plant 
of the same hybrid nature. And thus, the strange fact of 
the increase of fertility in the successive generations 
of artificially fertilised hybrids may, I believe, be ac- 
counted for by close interbreeding having been avoided. 
Now let us turn to the results arrived at by the third 
most experienced hybridiser, namely, the Hon. and 

M 3 


Kev. W. Herbert. He is as emphatic in his conclusion 
that some hybrids are perfectly fertile — as fertile as the 
pure parent-species — as are Kolreuter and Gartner that 
some degree of sterility between distinct species is a 
universal law of nature. He experimentised on some 
of the very same species as did Gartner. The differ- 
ence in their results may, I think, be in part ac- 
counted for by Herbert's great horticultural skill, and 
by his having hothouses at his command. Of his many 
important statements I will here give only a single 
one as an example, namely, that "every ovule in a 
pod of Crinum capense fertilised by C. revolutum pro- 
duced a plant, which (he says) I never saw to occur in 
a case of its natural fecundation." So that we here 
have perfect, or even more than commonly perfect, fer- 
tility in a first cross between two distinct species. 

This case of the Crinum leads me to refer to a most 
singular fact, namely, that there are individual plants, 
as with certain species of Lobelia, and with all the 
species of the genus Hippeastrum, which can be far more 
easily fertilised by the pollen of another and distinct 
species, than by their own pollen. For these plants 
have been found to yield seed to the pollen of a distinct 
species, though quite sterile with their own pollen, not- 
withstanding that their own pollen was found to be per- 
fectly good, for it fertilised distinct species. So that 
certain individual plants and all the individuals of cer- 
tain species can actually be hybridised much more 
readily than they can be self-fertilised ! For instance, 
a bulb of Hippeastrum aulicum produced four flowers ; 
three were fertilised by Herbert with their own pollen, 
and the fourth was subsequently fertilised by the pollen 
of a compound hybrid descended from three other and 
distinct species: the result was that "the ovaries of 
the three first flowers soon ceased to grow, and after a 


few days perished entirely, whereas the pod impreg- 
nated by the pollen of the hybrid made vigorous 
orowth and rapid progress to maturity, and bore good 
seed, which vegetated freely." In a letter to me m 
1839 Mr Herbert told me that he had then tried the 
experiment during five years, and he continued to try 
it during several subsequent years, and always with the 
same result. This result has, also, been confirmed by 
other observers in the case of Hippeastrum with its 
sub-genera, and in the case of some other genera, as 
Lobelia, Passiflora and Verbascum. Although the plants 
in these experiments appeared perfectly healthy, and 
although both the ovules and pollen of the same 
flower were perfectly good with respect to other species, 
yet as they were functionally imperfect in their mutual 
self-action, we must infer that the plants were in an 
unnatural state. Nevertheless these facts show on what 
slight and mysterious causes the lesser or greater fer- 
tility of species when crossed, in comparison with the 
same species when self-fertilised, sometimes depends. 

The practical experiments of horticulturists, though 
not made with scientific precision, deserve some 
notice. It is notorious in how complicated a manner 
the species of Pelargonium, Fuchsia, Calceolaria, Pe- 
tunia, Ehododendron, &c, have been crossed, yet many 
of these hybrids seed freely. For instance, Herbert 
asserts that a hybrid from Calceolaria mtegnfolia 
and plantaginea, species most widely dissimilar in 
general habit, "reproduced itself as perfectly as if 
it had been a natural species from the mountains ol 
Chile" I have taken some pains to ascertain the 
degree of fertility of some of the complex crosses of 
Khododendrons, and I am assured that many of them 
are perfectly fertile. Mr. C. Noble, for instance, informs 
me that he raises stocks for grafting from a hybrid 


between Khod. Ponticum and Catawbiense, and that 
this hybrid "seeds as freely as it is possible to ima- 
gine." Had hybrids, when fairly treated, gone on de- 
creasing in fertility in each successive generation, as 
Gartner believes to be the case, the fact would have 
been notorious to nurserymen. Horticulturists raise 
large beds of the same hybrids, and such alone are 
fairly treated, for by insect agency the several indi- 
viduals of the same hybrid variety are allowed to freely 
cross with each other, and the injurious influence of 
close interbreeding is thus prevented. Any one may 
readily convince himself of the efficiency of insect- 
agency by examining the flowers of the more sterile 
kinds of hybrid rhododendrons, which produce no pol- 
len, for he will find on their stigmas plenty of pollen 
brought from other flowers. 

In regard to animals, much fewer experiments have 
been carefully tried than with plants. If our systematic 
arrangements can be trusted, that is if the genera of 
animals are as distinct from each other, as are the genera 
of plants, then we may infer that animals more widely 
separated in the scale of nature can be more easily 
crossed than in the case of plants ; but the hybrids 
themselves are, I think, more sterile. I doubt whether 
any case of a perfectly fertile hybrid animal can be 
considered as thoroughly well authenticated. It should, 
however, be borne in mind that, owing to few animals 
breeding freely under confinement, few experiments 
have been fairly tried: for instance, the canary-bird 
has been crossed with nine other finches, but as not 
one of these nine species breeds freely in confinement, 
we have no right to expect that the first crosses be- 
tween them and the canary, or that their hybrids, 
should be perfectly fertile. Again, with respect to the 
fertility in successive generations of the more fertile 


hybrid animals, I hardly know of an instance in which 
two families of the same hybrid have been raised at the 
same time from different parents, so as to avoid the ill 
effects of close interbreeding. On the contrary, bro- 
thers and sisters have usually been crossed in each suc- 
cessive generation, in opposition to the constantly re- 
peated admonition of every breeder. And in this case, 
it is not at all surprising that the inherent sterility in 
the hybrids should have gone on increasing. If we 
were to act thus, and pair brothers and sisters in the 
case of any pure animal, which from any cause had 
the least tendency to sterility, the breed would as- 
suredly be lost in a very few generations. 

Although I do not know of any thoroughly well-authen- 
ticated cases of perfectly fertile hybrid animals, I have 
some reason to believe that the hybrids from Cervulus 
vaginalis and Keevesii, and from Phasianus colchicus 
with P. torquatus and with P. versicolor are perfectly 
fertile. The hybrids from the common and Chinese 
geese (A. cygnoides), species which are so different that 
they are generally ranked in distinct genera, have often 
bred in this country with either pure parent, and in 
one single instance they have bred inter se. This was 
effected by Mr. Eyton, who raised two hybrids from the 
same parents but from different hatches ; and from 
these two birds he raised no less than eight hybrids 
(grandchildren of the pure geese) from one nest. In 
India, however, these cross-bred geese must be far 
more fertile ; for I am assured by two eminently 
capable judges, namely Mr. Blyth and Capt, Hutton, 
that whole flocks of these crossed geese are kept in 
various parts of the country ; and as they are kept for 
profit, where neither pure parent-species exists, they 
must certainly be highly fertile. 

A doctrine which originated with Pallas, has been 


largely accepted by modern naturalists ; namely, that 
most of our domestic animals have descended from two 
or more aboriginal species, since commingled by inter- 
crossing. On this view, the aboriginal species must 
either at first have produced quite fertile hybrids, or 
the hybrids must have become in subsequent genera- 
tions quite fertile under domestication. This latter 
alternative seems to me the most probable, and I am 
inclined to believe in its truth, although it rests on no 
direct evidence. I believe, for instance, that our dogs 
have descended from several wild stocks ; yet, with per- 
haps the exception of certain indigenous domestic dogs 
of South America, all are quite fertile together; and 
analogy makes me greatly doubt, whether the several 
aboriginal species would at first have freely bred to- 
gether and have produced quite fertile hybrids. So 
again there is reason to believe that our European and 
the humped Indian cattle are quite fertile together ; 
but from facts communicated to me by Mr. Blyth, I 
think they must be considered as distinct species. On 
this view of the origin of many of our domestic animals, 
we must either give up the belief of the almost uni- 
versal sterility of distinct species of animals when 
crossed ; or we must look at sterility, not as an in- 
delible characteristic, but as one capable of being re- 
moved by domestication. 

Finally, looking to all the ascertained facts on the 
intercrossing of plants and animals, it may be con- 
cluded that some degree of sterility, both in first crosses 
and in hybrids, is an extremely general result ; but that 
it cannot, under our present state of knowledge, be con- 
sidered as absolutely universal. 

Laws governing the Sterility of first Crosses and of Hy- 
brids. — We will now consider a little more in detail the 


circumstances and rules governing the sterility of first 
crosses and of hybrids. Our chief object will be to see 
whether or not the rules indicate that species have 
specially been endowed with this quality, in order to 
prevent their crossing and blending together in utter 
confusion. The following rules and conclusions are 
chiefly drawn up from Gartner's admirable work on the 
hybridisation of plants. I have taken much pains to 
ascertain how far the rules apply to animals, and con- 
sidering how scanty our knowledge is in regard to hy- 
brid animals, I have been surprised to find how gene- 
rally the same rules apply to both kingdoms. 

It has been already remarked, that the degree of fer- 
tility, both of first crosses and of hybrids, graduates 
from zero to perfect fertility. It is surprising in how 
many curious ways this gradation can be shown to 
exist ; but only the barest outline of the facts can here 
be given. When pollen from a plant of one family is 
placed on the stigma of a plant of a distinct family, it 
exerts no more influence than so much inorganic dust. 
From this absolute zero of fertility, the pollen of differ- 
ent species of the same genus applied to the stigma of 
some one species, yields a perfect gradation in the 
number of seeds produced, up to nearly complete or 
even quite complete fertility; and, as we have seen, 
in certain abnormal cases, even to an excess of fertility, 
beyond that which the plant's own pollen will produce. 
So in hybrids themselves, there are some which never 
have produced, and probably never would produce, even 
with the pollen of either pure parent, a single fertile seed : 
but in some of these cases a first trace of fertility may 
be detected, by the pollen of one of the pure parent- 
species causing the flower of the hybrid to wither 
earlier than it otherwise would have done; and the 
early withering of the flower is well known to be a sign 


of incipient fertilisation. From this extreme degree 
of sterility we have self-fertilised hybrids producing a 
greater and greater number of seeds up to perfect fer- 

Hybrids from two species which are very difficult to 
cross, and which rarely produce any offspring, are gene- 
rally very sterile ; but the parallelism between the diffi- 
culty of making a first cross, and the sterility of the 
hybrids thus produced — two classes of facts which are 
generally confounded together — is by no means strict. 
There are many cases, in which two pure species can be 
united with unusual facility, and produce numerous 
hybrid-offspring, yet these hybrids are remarkably 
sterile. On the other hand, there are species which 
can be crossed very rarely, or with extreme difficulty, 
but the hybrids, when at last produced, are very fertile. 
Even within the limits of the same genus, for instance 
in Dianthus, these two opposite cases occur. 

The fertility, both of first crosses and of hybrids, is 
more easily affected by unfavourable conditions, than 
is the fertility of pure species. But the degree of 
fertility is likewise innately variable ; for it is not always 
the same when the same two species are crossed under 
the same circumstances, but depends in part upon the 
constitution of the individuals which happen to have 
been chosen for the experiment. So it is with hybrids, 
for their degree of fertility is often found to differ 
greatly in the several individuals raised from seed out 
of the same capsule and exposed to exactly the same 

By the t<.rm systematic affinity is meant, the resem- 
blance between species in structure and in constitution, 
more especially in the structure of parts which are of 
high physiological importance and which differ little in 
the allied species. Now the fertility of first crosses 


between species, and of the hybrids produced from 
them, is largely governed by their systematic af- 
finity. This is clearly shown by hybrids never having 
been raised between species ranked by systematists in 
distinct families ; and on the other hand, by very closely 
allied species generally uniting with facility. But the 
correspondence between systematic affinity and the 
facility of crossing is by no means strict. A mul- 
titude of cases could be given of very closely allied 
species which will not unite, or only with extreme 
difficulty ; and on the other hand of very distinct 
species which unite with the utmost facility. In the 
same family there may be a genus, as Dianthus, in 
which very many species can most readily be crossed ; 
and another genus, as Silene, in which the most 
persevering efforts have failed to produce between 
extremely close species a single hybrid. Even within 
the limits of the same genus, we meet with this same 
difference ; for instance, the many species of Nicotiana 
have been more largely crossed than the species of 
almost any other genus; but Gartner found that N. 
acuminata, which is not a particularly distinct species, 
obstinately failed to fertilise, or to be fertilised by, no 
less than eight other species of Nicotiana. Very many 
analogous facts could be given. 

No one has been able to point out what kind, or what 
amount, of difference in any recognisable character is 
sufficient to prevent two species crossing. It can be 
shown that plants most widely different in habit and 
general appearance, and having strongly marked differ- 
ences in every part of the flower, even in the pollen, in 
the fruit, and in the cotyledons, can be crossed. Annual 
and perennial plants, deciduous and evergreen trees, 
plants inhabiting different stations and fitted for ex- 
tremely different climates, can often be crossed with ease. 


By a reciprocal cross between two species, I mean 
the case, for instance, of a stallion-horse being first 
crossed with a female-ass, and then a male-ass with a 
mare : these two species may then be said to have been 
reciprocally crossed. There is often the widest possible 
difference in the facility of making reciprocal crosses. 
Such cases are highly important, for they prove that 
the capacity in any two species to cross is often com- 
pletely independent of their systematic affinity, or of 
any recognisable difference in their whole organisation. 
On the other hand, these cases clearly show that the 
capacity for crossing is connected with constitutional 
differences imperceptible by us, and confined to the 
reproductive system. This difference in the result of 
reciprocal crosses between the same two species was 
long ago observed by Kolreuter. To give an instance : 
Mirabilis jalappa can easily be fertilised by the pollen 
of M. longiflora, and the hybrids thus produced are 
sufficiently fertile ; but Kolreuter tried more than two 
hundred times, during eight following years, to fertilise 
reciprocally M. longiflora with the pollen of M. jalappa, 
and utterly failed. Several other equally striking cases 
could be given. Thuret has observed the same fact 
with certain sea-weeds or Fuci. Gartner, moreover, 
found that tins difference of facility in making reci- 
procal crosses is extremely common in a lesser degree. 
He has observed it even between forms so closely related 
(as Matthiola annua and glabra) that many botanists 
rank them only as varieties. It is also a remarkable 
fact, that hybrids raised from reciprocal crosses, though 
of course compounded of the very same two species, 
the one species having first been used as the father and 
then as the mother, generally differ in fertility in a 
small, and occasionally in a high degree. 

Several other singular rules could be given from 


Gartner : for instance, some species have a remarkable 
power of crossing with other species ; other species of 
the same genus have a remarkable power of impressing 
their likeness on their hybrid offspring; but these 
two powers do not at all necessarily go together. 
There are certain hybrids which instead of having, as 
is usual, an intermediate character between their two 
parents, always closely resemble one of them ; and 
such hybrids, though externally so like one of their pure 
parent-species, are with rare exceptions extremely 
sterile. So again amongst hybrids which are usually 
intermediate in structure between their parents, ex- 
ceptional and abnormal individuals sometimes are born, 
which closely resemble one of their pure parents ; and 
these hybrids are almost always utterly sterile, even 
when the other hybrids raised from seed from the same 
capsule have a considerable degree of fertility. These 
facts show how completely fertility in the hybrid is 
independent of its external resemblance to either pure 

Considering the several rules now given, which 
govern the fertility of first crosses and of hybrids, we 
see that when forms, which must be considered as 
good and distinct species, are united, their fertility 
graduates from zero to perfect fertility, or even to 
fertility under certain conditions in excess. That 
their fertility, besides being eminently susceptible to 
favourable and unfavourable conditions, is innately 
variable. That it is by no means always the same in 
degree in the first cross and in the hybrids produced 
from this cross. That the fertility of hybrids is not 
related to the degree in which they resemble in exter- 
nal appearance either parent. And lastly, that the 
facility of making a first cross between any two species 
is not always governed by their systematic affinity or 


degree of resemblance to each other. This latter 
statement is clearly proved by reciprocal crosses 
between the same two species, for according as the 
one species or the other is used as the father or 
the mother, there is generally some difference, and 
occasionally the widest possible difference, in the 
facility of effecting an union. The hybrids, more- 
over, produced from reciprocal crosses often differ in 

Now do these complex and singular rules indicate 
that species have been endowed with sterility simply 
to prevent their becoming confounded in nature? I 
think not. For why should the sterility be so extremely 
different in degree, when various species are crossed, all 
of which we must suppose it would be equally important 
to keep from blending together? Why should the 
degree of sterility be innately variable in the individuals 
of the same species ? Why should some species cross 
with facility, and yet produce very sterile hybrids ; 
and other species cross with extreme difficulty, and yet 
produce fairly fertile hybrids ? Why should there often 
be so great a difference in the result of a reciprocal 
cross between the same two species? Why, it may 
even be asked, has the production of hybrids been per- 
mitted ? to grant to species the special power of pro- 
ducing hybrids, and then to stop then further propaga- 
tion by different degrees of sterility, not strictly related 
to the facility of the first union between their parents, 
seems to be a strange arrangement. 

The foregoing rules and facts, on the other hand, ap- 
pear to me clearly to indicate that the sterility both of 
first crosses and of hybrids is simply incidental or de- 
pendent on unknown differences, chiefly in the repro- 
ductive systems, of the species which are crossed. The 
differences being of so peculiar and limited a nature, 



that, in reciprocal crosses between two species the male 
sexual element of the one will often freely act on the 
female sexual element of the other, but not in a re- 
versed direction. It will be advisable to explain a little 
more fully by an example what I mean by sterility 
being incidental on other differences, and not a speci- 
ally endowed quality. As the capacity of one plant to 
be grafted or budded on another is so entirely unim- 
portant for its welfare in a state of nature, I presume 
that no one will suppose that this capacity is a specially 
endowed quality, but will admit that it is incidental 
on differences in the laws of growth of the two plants. 
We can sometimes see the reason why one tree will 
not take on another, from differences in their rate of 
growth, in the hardness of their wood, in the period of 
the flow or nature of their sap, &c. ; but in a multitude 
of cases we can assign no reason whatever. Great di- 
versity in the size of two plants, one being woody and 
the other herbaceous, one being evergreen and the 
other deciduous, and adaptation to widely different 
climates, does not always prevent the two grafting to- 
gether. As in hybridisation, so with grafting, the 
capacity is limited by systematic affinity, for no one 
has been able to graft trees together belonging to quite 
distinct families ; and, on the other hand, closely allied 
species, and varieties of the same species, can usually, 
but not invariably, be grafted with ease. But this capa- 
city, as in hybridisation, is by no means absolutely go- 
verned by systematic affinity. Although many distinct 
genera within the same family have been grafted to- 
gether, in other cases species of the same genus will 
not take on each other. The pear can be grafted far 
more readily on the quince, which is ranked as a distinct 
genus, than on the apple, which is a member of the 
same genus. Even different varieties of the pear take 


with different degrees of facility on the quince ; so do 
different varieties of the apricot and peach on certain 
varieties of the plum. 

As Gartner found that there was sometimes an innate 
difference in different individuals of the same two spe- 
cies in crossing ; so Sagaret believes this to be the case 
with different individuals of the same two species in 
being grafted together. As in reciprocal crosses, the 
facility of effecting an union is often very far from 
equal, so it sometimes is in grafting; the common 
gooseberry, for instance, cannot be grafted on the cur- 
rant, whereas the currant will take, though with diffi- 
culty, on the gooseberry. 

We have seen that the sterility of hybrids, which 
have their reproductive organs in an imperfect con- 
dition, is a very different case from the difficulty of 
uniting two pure species, which have their reproduc- 
tive organs perfect ; yet these two distinct cases run 
to a certain extent parallel. Something analogous 
occurs in grafting ; for Thouin found that three species 
of Kobinia, which seeded freely on their own roots, 
and which could be grafted with no great difficulty 
on another species, when thus grafted were rendered 
barren. On the other hand, certain species of Sorbus, 
when grafted on other species, yielded twice as much 
fruit as when on their own roots. We are reminded by 
this latter fact of the extraordinary case of Hippe- 
astrum, Lobelia, &c, which seeded much more freely 
when fertilised with the pollen of distinct species, than 
when self-fertilised with their own pollen. 

W"e thus see, that although there is a clear and 
fundamental difference between the mere adhesion of 
grafted stocks, and the union of tfie'niale and female 
elements in the act of reproduction, yet that there is a 
rude degree of parallelism in the results of grafting and 


of crossing distinct species. And as we must look at 
the curious and complex laws governing the facility with 
which trees can be grafted on each other as incidental 
on unknown differences in their vegetative systems, so 
I believe that the still more complex laws governing 
the facility of first crosses, are incidental on unknown 
differences, chiefly in their reproductive systems. 
These differences, in both cases, follow to a certain 
extent, as might have been expected, systematic affinity, 
by which every kind of resemblance and dissimilarity 
between organic beings is attempted to be expressed. 
The facts by no means seem to me to indicate that the 
greater or lesser difficulty of either grafting or crossing 
together various species has been a special endow- 
ment ; although in the case of crossing, the difficulty 
is as important for the endurance and stability of spe- 
cific forms, as in the case of grafting it is unimportant 
for their welfare. 

Causes of the Sterility of first Crosses and of Hybrids. — 
We may now look a little closer at the probable causes 
of the sterility of first crosses and of hybrids. These 
two cases are fundamentally different, for, as just 
remarked, in the union of two pure species the male 
and female sexual elements are perfect, whereas in 
hybrids they are imperfect. Even in first crosses, the 
greater or lesser difficulty in effecting a union apparently 
depends on several distinct causes. There must some- 
times be a physical impossibility in the male element 
reaching the ovule, as would be the case with a plant 
having a pistil too long for the pollen-tubes to reach the 
ovarium. It has also been observed that when pollen 
of one species is placed on the stigma of a distantly 
allied species, though the pollen-tubes protrude, they 
do not penetrate the stigmatic surface. Again, the 


male element may reach the female element, but be 
incapable of causing an embryo to be developed, as 
seems to have been the case with some of Thuret's 
experiments on Fuci. No explanation can be given 
of these facts, any more than why certain trees cannot 
be grafted on others. Lastly, an embryo may be 
developed, and then perish at an early period. This 
latter alternative has not been sufficiently attended to ; 
but I believe, from observations communicated to me by 
Mr. Hewitt, who has had great experience in hybridising 
gallinaceous birds, that the early death of the embryo 
is a very frequent cause of sterility in first crosses. I 
was at first very unwilling to believe in tins view; 
as hybrids, when once born, are generally healthy 
and long-lived, as we see in the case of the common 
mule. Hybrids, however, are differently circumstanced 
before and after birth : when born and living in a coun- 
try where their two parents can live, they are gene- 
rally placed under suitable conditions of life. But a 
hybrid partakes of only half of the nature and consti- 
tution of its mother, and therefore before birth, as long 
as it is nourished within its mother's womb or within 
the egg or seed produced by the mother, it may be 
exposed to conditions in some degree unsuitable, 
and consequently be liable to perish at an early 
period ; more especially as all very young beings seem 
eminently sensitive to injurious or unnatural condi- 
tions of life. 

In regard to the sterility of hybrids, in which the 
sexual elements are imperfectly developed, the case is 
very different. I have more than once alluded to a 
large body of facts, which I have collected, showing 
that when animals and plants are removed from their 
natural conditions, they are extremely liable to have their 
reproductive systems seriously affected. Tins, in fact, is 


the great bar to the domestication of animals. Between 
the sterility thus superinduced and that of hybrids, there 
are many points of similarity. In both cases the sterility 
is independent of general health, and is often accom- 
panied by excess of size or great luxuriance. In both 
cases, the sterility occurs in various degrees; in both, 
the male element is the most liable to be affected ; but 
sometimes the female more than the male. In both, 
the tendency goes to a certain extent with systematic 
affinity, for whole groups of animals and plants are ren- 
dered impotent by the same unnatural conditions ; and 
whole groups of species tend to produce sterile hybrids. 
On the other hand, one species in a group will some- 
times resist great changes of conditions with unimpaired 
fertility; and certain species in a group will produce 
unusually fertile hybrids. No one can tell, till he tries, 
whether any particular animal will breed under confine- 
ment or any plant seed freely under culture ; nor can 
he tell, till he tries, whether any two species of a genus 
will produce more or less sterile hybrids. Lastly, when 
organic beings are placed during several generations 
under conditions not natural to them, they are ex- 
tremely liable to vary, which is due, as I believe, 
to their reproductive systems having been specially 
affected, though in a lesser degree than when sterility 
ensues. So it is with hybrids, for hybrids in successive 
generations are eminently liable to vary, as every expe- 
rimentalist has observed. 

Thus we see that when organic beings are placed 
under new and unnatural conditions, and when hybrids 
are produced by the unnatural crossing of two species, 
the reproductive system, independently of the general 
state of health, is affected by sterility in a very similar 
manner. In the one case, the conditions of life have 
been disturbed, though often in so slight a degree as to 



be inappreciable by us ; in the other case, or that of 
hybrids, the external conditions have remained the same, 
but the organisation has been disturbed by two different 
structures and constitutions having been blended into 
one. For it is scarcely possible that two organisations 
should be compounded into one, without some disturb- 
ance occurring in the development, or periodical action, 
or mutual relation of the different parts and organs one 
to another, or to the conditions of life. When hybrids 
are able to breed inter se, they transmit to their offspring 
from generation to generation the same compounded 
organisation, and hence we need not be surprised that 
their sterility, though in some degree variable, rarely 

It must, however, be confessed that we cannot under- 
stand, excepting on vague hypotheses, several facts with 
respect to the sterility of hybrids ; for instance, the 
unequal fertility of hybrids produced from reciprocal 
crosses ; or the increased sterility in those hybrids which 
occasionally and exceptionally resemble closely either 
pure parent. Nor do I pretend that the foregoing 
remarks go to the root of the matter : no explanation 
is offered why an organism, when placed under unna- 
tural conditions, is rendered sterile. All that I have 
attempted to show, is that in two cases, in some respects 
allied, sterility is the common result, — in the one case 
from the conditions of life having been disturbed, in the 
other case from the organisation having been disturbed 
by two organisations having been compounded into one. 

It may seem fanciful, but I suspect that a similar 
parallelism extends to an allied yet very different class 
of facts. It is an old and almost universal belief, 
founded, I think, on a considerable body of evidence, 
that slight changes in the conditions of life are bene- 
ficial to all living things. We see this acted on by 


farmers and gardeners in their frequent exchanges of 
seed, tubers, &c, from one soil or climate to another, 
and back again. During the convalescence of animals, 
we plainly see that great benefit is derived from almost 
any change in the habits of life. Again, both with 
plants and animals, there is abundant evidence, that a 
cross between very distinct individuals of the same spe- 
cies, that is between members of different strains or 
sub-breeds, gives vigour and fertility to the offspring. 
I believe, indeed, from the facts alluded to in our fourth 
chapter, that a certain amount of crossing is indispens- 
able even with hermaphrodites; and that close inter- 
breeding continued during several generations between 
the nearest relations, especially if these be kept under 
the same conditions of life, always induces weakness 
and sterility in the progeny. 

Hence it seems that, on the one hand, slight changes 
in the conditions of life benefit all organic beings, and 
on the other hand, that slight crosses, that is crosses 
between the males and females of the same species 
which have varied and become slightly different, give 
vigour and fertility to the offspring. But we have 
seen that greater changes, or changes of a particular 
nature, often render organic beings in some degree 
sterile ; and that greater crosses, that is crosses between 
males and females which have become widely or spe- 
cifically different, produce hybrids which are generally 
sterile in some degree. I cannot persuade myself that 
this parallelism is an accident or an illusion. Both 
series of facts seem to be connected together by some 
common but unknown bond, which is essentially related 
to the principle of life. 

Fertility of Varieties when crossed, and of their Mongrel 
offspring. — It may be urged, as a most forcible argu- 



ment, that there must be some essential distinction 
between species and varieties, and that there must be 
some error in all the foregoing remarks, inasmuch as 
varieties, however much they may differ from each 
other in external appearance, cross with perfect facility, 
and yield perfectly fertile offspring. I fully admit that 
this is almost invariably the case. But if we look to 
varieties produced under nature, we are immediately 
involved in hopeless difficulties ; for if two hitherto 
reputed varieties be found in any degree sterile to- 
gether, they are at once ranked by most naturalists 
as species. For instance, the blue and red pimpernel, 
the primrose and cowslip, which are considered by 
many of our best botanists as varieties, are said by 
Gartner not to be quite fertile when crossed, and he 
consequently ranks them as undoubted species. If we 
thus argue in a circle, the fertility of all varieties pro- 
duced under nature will assuredly have to be granted. 

If we turn to varieties, produced, or supposed to have 
been produced, under domestication, v^e are still in- 
volved in doubt. For when it is stated, for instance, 
that the German Spitz dog unites more easily than 
other dogs with foxes, or that certain South American 
indigenous domestic dogs do not readily cross with Euro- 
pean dogs, the explanation which will occur to every 
one, and probably the true one, is that these dogs have 
descended from several aboriginally distinct species. 
Nevertheless the perfect fertility of so many domestic 
varieties, differing widely from each other in appear- 
ance, for instance of the pigeon or of the cabbage, is 
a remarkable fact ; more especially when we reflect 
how many species there are, which, though resem- 
bling each other most closely, are utterly sterile when 
intercrossed. Several considerations, however, render 
the fertility of domestic varieties less remarkable than 


at first appears. It can, in the first place, be clearly 
shown that mere external dissimilarity between two spe- 
cies does not determine their greater or lesser degree of 
sterility when crossed ; and we may apply the same rule 
to domestic varieties. In the second place, some emi- 
nent naturalists believe that a long course of domesti- 
cation tends to eliminate sterility in the successive 
generations of hybrids, which were at first only slightly 
sterile ; and if this be so, we surely ought not to expect 
to find sterility both appearing and disappearing under 
nearly the same conditions of life. Lastly, and this 
seems to me by far the most important consideration, 
new races of animals and plants are produced under 
domestication by man's methodical and unconscious 
power of selection, for his own use and pleasure : he 
neither wishes to select, nor could select, slight differ- 
ences in the reproductive system, or other constitutional 
differences correlated with the reproductive system. 
He supplies his several varieties with the same food ; 
treats them in nearly the same manner, and does not 
wish to alter their general habits of life. Nature acts 
uniformly and slowly during vast periods of time on the 
whole organisation, in any way which may be for each 
creature's own good ; and thus she may, either directly, 
or more probably indirectly, through correlation, modify 
the reproductive system in the several descendants from 
any one species. Seeing this difference in the process 
of selection, as carried on by man and nature, we need 
not be surprised at some difference in the result. 

I have as yet spoken as if the varieties of the same 
species were invariably fertile when intercrossed. But 
it seems to me impossible to resist the evidence of the 
existence of a certain amount of sterility in the few 
following cases, which I will briefly abstract. The evi- 
dence is at least as good as that from which we believe 


270 HYBRIDISM. Chap. VJ1I. 

in the sterility of a multitude of species. The evidence 
is, also, derived from hostile witnesses, who in all other 
cases consider fertility and sterility as safe criterions 
of specific distinction. Gartner kept during several 
years a dwarf kind of maize with yellow seeds, and a 
tall variety with red seeds, growing near each other in 
his garden ; and although these plants have separated 
sexes, they never naturally crossed. He then fertilised 
thirteen flowers of the one with the pollen of the other ; 
but only a single head produced any seed, and this one 
head produced only five grains. Manipulation in this 
case could not have been injurious, as the plants have 
separated sexes. No one, I believe, has suspected that 
these varieties of maize are distinct species; and 
it is important to notice that the hybrid plants thus 
raised were themselves 'perfectly fertile ; so that even 
Gartner did not venture to consider the two varieties as 
specifically distinct. 

Girou de Buzareingues crossed three varieties of 
gourd, which like the maize has separated sexes, and 
he asserts that their mutual fertilisation is by so much 
the less easy as their differences are greater. How far 
these experiments may be trusted, I know not ; but the 
forms experimented on, are ranked by Sagaret, who 
mainly founds his classification by the test of infertility, 
as varieties. 

The following case is far more remarkable, and seems 
at first quite incredible ; but it is the result of an asto- 
nishing number of experiments made during many years 
on nine species of Verbascum, by so good an observer 
and so hostile a witness, as Gartner : namely, that yellow 
and white varieties of the same species of Verbascum 
when intercrossed produce less seed, than do either 
coloured varieties when fertilised with pollen from their 
own coloured flowers. Moreover, he asserts that when 


yellow and white varieties of one species are crossed 
with yellow and white varieties of a distinct species, 
more seed is produced by the crosses between the same 
coloured flowers, than between those which are differ- 
ently coloured. Yet these varieties of Verbascum pre- 
sent no other difference besides the mere colour of the 
flower ; and one variety can sometimes be raised from 
the seed of the other. 

From observations which I have made on certain 
varieties of hollyhock, I am inclined to suspect that 
tney present analogous facts. 

Kolreuter, whose accuracy has been confirmed by 
every subsequent observer, has proved the remarkable 
fact, that one variety of the common tobacco is more 
fertile, when crossed with a widely distinct species, 
than are the other varieties. He experimentised on five 
forms, which are commonly reputed to be varieties, and 
which he tested by the severest trial, namely, by reci- 
procal crosses, and he found their mongrel offspring 
perfectly fertile. But one of these five varieties, when 
used either as father or mother, and crossed with the 
Nicotiana glutinosa, always yielded hybrids not so 
sterile as those winch were produced from the four 
other varieties when crossed with N. glutinosa. Hence 
the reproductive system of this one variety must have 
been in some manner and in some degree modified. 

From these facts ; from the great difficulty of ascer- 
taining the infertility of varieties in a state of nature, 
for a supposed variety if infertile in any degree would 
generally be ranked as species ; from man selecting only 
external characters in the production of the most dis- 
tinct domestic varieties, and from not wishing or being 
able to produce recondite and functional differences in 
the reproductive system; from these several consider- 
ations and facts, I do not think that the very general 



fertility of varieties can be proved to be of universal 
occurrence, or to form a fundamental distinction between 
varieties and species. The general fertility of varieties 
does not seem to me sufficient to overthrow the view 
which I have taken with respect to the very general, 
but not invariable, sterility of first crosses and of hybrids, 
namely, that it is not a special endowment, but is inci- 
dental on slowly acquired modifications, more especially 
in the reproductive systems of the forms which are 

Hybrids and Mongrels compared, independently of their 
fertility. — Independently of the question of fertility, the 
offspring of species when crossed and of varieties when 
crossed may be compared in several other respects. 
Gartner, whose strong wish was to draw a marked line 
of distinction between species and varieties, could find 
very few and, as it seems to me, quite unimportant 
differences between the so-called hybrid offspring of 
species, and the so-called mongrel offspring of varieties. 
And, on the other hand, they agree most closely in very 
many important respects. 

I shall here discuss this subject with extreme brevity. 
The most important distinction is, that in the first 
generation mongrels are more variable than hybrids ; 
but Gartner admits that hybrids from species which 
have long been cultivated are often variable in the first 
generation ; and I have myself seen striking instances 
of this fact. Gartner further admits that hybrids be- 
tween very closely allied species are more variable 
than those from very distinct species ; and this shows 
that the difference in the degree of variability gra- 
duates away. When mongrels and the more fertile 
hybrids are propagated for several generations an ex- 
treme amount of variability in their offspring is notori- 


ous ; but some few cases both of hybrids and mongrels 
long retaining uniformity of character could be given. 
The variability, however, in the successive generations 
of mongrels is, perhaps, greater than in hybrids. 

This greater variability of mongrels than of hybrids 
does not seem to me at all surprising. For the parents 
of mongrels are varieties, and mostly domestic varieties 
(very few experiments having been tried on natural 
varieties), and this implies in most cases that there has 
been recent variability ; and therefore we might expect 
that such variability would often continue and be super- 
added to that arising from the mere act of crossing. 
The slight degree of variability in hybrids from the first 
cross or in the first generation, in contrast with their 
extreme variability in the succeeding generations, is a 
curious fact and deserves attention. For it bears on 
and corroborates the view which I have taken on the 
cause of ordinary variability ; namely, that it is due 
to the reproductive system being eminently sensitive 
to any change in the conditions of life, being thus 
often rendered either impotent or at least incapable 
of its proper function of producing offspring identical 
with the parent-form. Now hybrids in the first gene- 
ration are descended from species (excluding those 
long cultivated) which have not had their repro- 
ductive systems in any way affected, and they are 
not variable ; but hybrids themselves have their repro- 
ductive systems seriously affected, and their descend- 
ants are highly variable. 

But to return to our comparison of mongrels and 
hybrids : Gartner states that mongrels are more liable 
than hybrids to revert to either parent-form ; but 
this, if it be true, is certainly only a difference in 
degree. Gartner further insists that when any two 
species, although most closely allied to each other, are 

N 3 

274 HYBE1DISM. Chap. VIII. 

crossed with a third species, the hybrids are widely 
different from each other ; whereas if two very distinct 
varieties of one species are crossed with another species, 
the hybrids do not differ much. But this conclusion, 
as far as I can make out, is founded on a single experi- 
ment ; and seems directly opposed to the results of 
several experiments made by Kolreuter. 

These alone are the unimportant differences, which 
Gartner is able to point out, between hybrid and 
mongrel plants. On the other hand, the resemblance 
in mongrels and in hybrids to their respective parents, 
more especially in hybrids produced from nearly re- 
lated species, follows according to Gartner the same 
laws. When two species are crossed, one has some- 
times a prepotent power of impressing its likeness 
on the hybrid ; and so I believe it to be with varieties 
of plants. With animals one variety certainly often has 
this prepotent power over another variety. Hybrid 
plants produced from a reciprocal cross, generally re- 
semble each other closely ; and so it is with mongrels 
from a reciprocal cross. Both hybrids and mongrels 
can be reduced to either pure parent-form, by repeated 
crosses in successive generations with either parent. 

These several remarks are apparently applicable to 
animals ; but the subject is here excessively compli- 
cated, partly owing to the existence of secondary sexual 
characters ; but more especially owing to prepotency 
in transmitting likeness running more strongly in one 
sex than in the other, both when one species is crossed 
with another, and when one variety is crossed with an- 
other variety. For instance, I think those authors are 
right, who maintain that the ass has a prepotent power 
over the horse, so that both the mule and the hinny 
more resemble the ass than the horse ; but that the 
prepotency runs more strongly in the male-ass than in 


the female, so that the mule, which is the offspring of 
the male-ass and mare, is more like an ass, than is the 
hinny, which is the offspring of the female-ass and 

Much stress has been laid by some authors on the 
supposed fact, that mongrel animals alone are born 
closely like one of their parents ; but it can be shown 
that this does sometimes occur with hybrids; yet I 
grant much less frequently with hybrids than with 
mongrels. Looking to the cases which I have collected 
of cross-bred animals closely resembling one parent, 
the resemblances seem chiefly confined to characters 
almost monstrous in their nature, and which have sud- 
denly appeared — such as albinism, melanism, deficiency 
of tail or horns, or additional fingers and toes ; and do 
not relate to characters which have been slowly acquired 
by selection. Consequently, sudden reversions to the 
perfect character of either parent would be more likely 
to occur with mongrels, which are descended from va- 
rieties often suddenly produced and semi-monstrous in 
character, than with hybrids, which are descended from 
species slowly and naturally produced. On the whole 
I entirely agree with Dr. Prosper Lucas, who, after 
arranging an enormous body of facts with respect to 
animals, comes to the conclusion, that the laws of resem- 
blance of the child to its parents are the same, whether 
the two parents differ much or little from each other, 
namely in the union of individuals of the same variety, 
or of different varieties, or of distinct species. 

Laying aside the question of fertility and sterility, 
in all other respects there seems to be a general and 
close similarity in the offspring of crossed species, and of 
crossed varieties. If we look at species as having been 
specially created, and at varieties as having been pro- 
duced by secondary laws, this similarity would be an 


astonishing fact. But it harmonises perfectly with the 
view that there is no essential distinction between spe- 
cies and varieties. 

Summary of Chapter. — First crosses between forms 
sufficiently distinct to be ranked as species, and their 
hybrids, are very generally, but not universally, sterile. 
The sterility is of all degrees, and is often so slight that 
the two most careful experimentalists who have ever 
lived, have come to diametrically opposite conclusions 
in ranking forms by this test. The sterility is innately 
variable in individuals of the same species, and is 
eminently susceptible of favourable and unfavourable 
conditions. The degree of sterility does not strictly 
follow systematic affinity, but is governed by several 
curious and complex laws. It is generally different, 
and sometimes widely different, in reciprocal crosses 
between the same two species. It is not always equal 
in degree in a first cross and in the hybrid produced 
from this cross. 

In the same manner as in grafting trees, the capacity 
of one species or variety to take on another, is incidental 
on generally unknown differences in their vegetative 
systems, so in crossing, the greater or less facility of one 
species to unite with another, is incidental on unknown 
differences in their reproductive systems. There is no 
more reason to think that species have been specially 
endowed with various degrees of sterility to prevent 
them crossing and blending in nature, than to tliink 
that trees have been specially endowed with various and 
somewhat analogous degrees of difficulty in being grafted 
together in order to prevent them becoming inarched 
in our forests. 

The sterility of first crosses between pure species, 
which have their reproductive systems perfect, seems 

Chap. VIII. SUMMARY. 277 

to depend on several circumstances ; in some cases 
largely on the early death of the embryo. The sterility 
of hybrids, which have their reproductive systems im- 
perfect, and which have had this system and their whole 
organisation disturbed by being compounded of two dis- 
tinct species, seems closely allied to that sterility which 
so frequently affects pure species, when their natural 
conditions of life have been disturbed. This view is 
supported by a parallelism of another kind ; — namely, 
that the crossing of forms only slightly different is 
favourable to the vigour and fertility of their offspring ; 
and that slight changes in the conditions of life are ap- 
parently favourable to the vigour and fertility of all 
organic beings. It is not surprising that the degree of 
difficulty in uniting two species, and the degree of 
sterility of their hybrid-offspring should generally cor- 
respond, though due to distinct causes ; for both depend 
on the amount of difference of some kind between the 
species which are crossed. Nor is it surprising that 
the facility of effecting a first cross, the fertility of the 
hybrids produced, and the capacity of being grafted to- 
gether — though this latter capacity evidently depends 
on widely different circumstances — should all run, to a 
certain extent, parallel with the systematic affinity of 
the forms which are subjected to experiment ; for sys- 
tematic affinity attempts to express all kinds of resem- 
blance between all species. 

First crosses between forms known to be varieties, or 
sufficiently alike to be considered as varieties, and their 
mongrel offspring, are very generally, but not quite uni- 
versally, fertile. Nor is this nearly general and perfect 
fertility sui*prising, when we remember how liable we are 
to argue in a circle with respect to varieties in a state 
of nature ; and when we remember that the greater 
number of varieties have been produced under domesti- 


cation by the selection of mere external differences, and 
not of differences in the reproductive system. In all 
other respects, excluding fertility, there is a close gene- 
ral resemblance between hybrids and mongrels. Finally, 
then, the facts briefly given in this chapter do not seem 
to me opposed to, but even rather to support the view, 
that there is no fundamental distinction between species 
and varieties. 



On the Imperfection of the Geological Record. 

On the absence of intermediate varieties at the present day — On 
the nature of extinct intermediate varieties ; on their number — 
On the vast lapse of time, as inferred from the rate of deposi- 
tion and of denudation — On the poorness of our palEeontological 
collections — On the intermittence of geological formations — 
On the absence of intermediate varieties in any one formation 
— On the sudden appearance of groups of species — On their 
sudden appearance in the lowest known fossiliferous strata. 

In the sixth chapter I enumerated the chief objections 
which might be justly urged against the views main- 
tained in this volume. Most of them have now been 
discussed. One, namely the distinctness of specific 
forms, and their not being blended together by innu- 
merable transitional links, is a very obvious difficulty. 
I assigned reasons why such links do not commonly 
occur at the present day, under the circumstances ap- 
parently most favourable for their presence, namely 
on an extensive and continuous area with graduated 
physical conditions. I endeavoured to show, that the 
life of each species depends in a more important manner 
on the presence of other already defined organic forms, 
than on climate; and, therefore, that the really go- 
verning conditions of life do not graduate away quite 
insensibly like heat or moisture. I endeavoured, also, 
to show that intermediate varieties, from existing in 
lesser numbers than the forms which they connect, will 
generally be beaten out and exterminated during the 
course of further modification and improvement. The 
main cause, however, of innumerable intermediate links 
not now occurring everywhere throughout nature de- 


pends on the very process of natural selection, through 
which new varieties continually take the places of and 
exterminate their parent-forms. But just in proportion 
as this process of extermination has acted on an 
enormous scale, so must the number of intermediate 
varieties, which have formerly existed on the earth, 
be truly enormous. Why then is not every geo- 
logical formation and every stratum full of such inter- 
mediate links ? Geology assuredly does not reveal any 
such finely graduated organic chain ; and this, perhaps, 
is the most obvious and gravest objection which can be 
urged against my theory. The explanation lies, as I 
believe, in the extreme imperfection of the geological 

In the first place it should always be borne in mind 
what sort of intermediate forms must, on my theory, 
have formerly existed. I have found it difficult, when 
looking at any two species, to avoid picturing to myself, 
forms directly intermediate between them. But this 
is a wholly false view ; we should always look for forms 
intermediate between each species and a common but 
unknown progenitor ; and the progenitor will generally 
have differed in some respects from all its modified de- 
scendants. To give a simple illustration : the fantail and 
pouter pigeons have both descended from the rock-pigeon ; 
if we possessed all the intermediate varieties which 
have ever existed, we should have an extremely close series 
between both and the rock-pigeon ; but we should have 
no varieties directly intermediate between the fantail 
and pouter ; none, for instance, combining a tail some- 
what expanded with a crop somewhat enlarged, the 
characteristic features of these two breeds. These two 
breeds, moreover, have become so much modified, that 
if we had no historical or indirect evidence regarding 
their origin, it would not have been possible to have 


determined from a mere comparison of their structure 
with that of the rock-pigeon, whether they had descended 
from this species or from some other allied species, such 
as C. oenas. 

So with natural species, if we look to forms very 
distinct, for instance to the horse and tapir, we have 
no reason to suppose that links ever existed directly 
intermediate between them, but between each and an 
unknown common parent. The common parent will 
have had in its whole organisation much general resem- 
blance to the tapir and to the horse ; but in some points 
of structure may have differed considerably from both, 
even perhaps more than they differ from each other. 
Hence in all such cases, we should be unable to recog- 
nise the parent-form of any two or more species, even 
if we closely compared the structure of the parent with 
that of its modified descendants, unless at the same 
time we had a nearly perfect chain of the intermediate 

It is just possible by my theory, that one of two living 
forms might have descended from the other; for in- 
stance, a horse from a tapir ; and in this case direct 
intermediate links will have existed between them. 
But such a case would imply that one form had re- 
mained for a very long period unaltered, whilst its 
descendants had undergone a vast amount of change ; 
and the principle of competition between organism and 
organism, between child and parent, will render this a 
very rare event ; for in all cases the new and improved 
forms of life will tend to supplant the old and unim- 
proved forms. 

By the theory of natural selection all living species 
have been connected with the parent-species of each 
genus, by differences not greater than we see be- 
tween the varieties of the same species at the present 


day ; and these parent-species, now generally extinct, 
have in their turn been similarly connected with more 
ancient species; and so on backwards, always con- 
verging to the common ancestor of each great class. 
So that the number of intermediate and transitional 
links, between all living and extinct species, must have 
been inconceivably great. But assuredly, if this theory 
be true, such have lived upon this earth. 

On the lapse of Time. — Independently of our not 
finding fossil remains of such infinitely numerous con- 
necting links, it may be objected, that time will not 
have sufficed for so great an amount of organic change, 
all changes having been effected very slowly through 
natural selection. It is hardly possible for me even to 
recall to the reader, who may not be a practical geo- 
logist, the facts leading the mind feebly to comprehend 
the lapse of time. He who can read Sir Charles Lyell's 
grand work on the Principles of Geology, winch the 
future historian will recognise as having produced a 
revolution in natural science, yet does not admit how 
incomprehensibly vast have been the past periods of 
time, may at once close tins volume. Not that it suffices 
to study the Principles of Geology, or to read special 
treatises by different observers on separate formations, 
and to mark how each author attempts to give an in- 
adequate idea of the duration of each formation or even 
each stratum. A man must for years examine for 
himself great piles of superimposed strata, and watch 
the sea at work grinding down old rocks and making 
fresh sediment, before he can hope to comprehend any- 
thing of the lapse of time, the monuments of which we 
see around us. 

It is good to wander along lines of sea-coast. 
when formed of moderately hard rocks, and mark the 


process of degradation. The tides in most cases reach 
the cliffs only for a short time twice a day, and the 
waves eat into them only when they are charged 
with sand or pebbles; for there is reason to believe 
that pure water can effect little or nothing in wearing 
away rock. At last the base of the cliff is under- 
mined, huge fragments fall down, and these remain- 
ing fixed, have to be worn away, atom by atom, until 
reduced in size they can be rolled about by the waves, 
and then are more quickly ground into pebbles, sand, 
or mud. But how often do we see along the bases of 
retreating cliffs rounded boulders, all thickly clothed 
by marine productions, showing how little they are 
abraded and how seldom they are rolled about ! More- 
over, if we follow for a few miles any line of rocky cliff, 
which is undergoing degradation, we find that it is only 
here and there, along a short length or round a pro- 
montory, that the cliffs are at the present time suffering. 
The appearance of the surface and the vegetation show 
that elsewhere years have elapsed since the waters 
washed their base. 

He who most closely studies the action of the sea on 
our shores, will, I believe, be most deeply impressed 
with the slowness with which rocky coasts are worn 
away. The observations on this head by Hugh Miller, 
and by that excellent observer Mr. Smith of Jordan 
Hill, are most impressive. With the mind thus im- 
pressed, let any one examine beds of conglomerate 
many thousand feet in thickness, which, though pro- 
bably formed at a quicker rate than many other depo- 
sits, yet, from being formed of worn and rounded 
pebbles, each of which bears the stamp of time, are 
good to show how slowly the mass has been accumu- 
lated. Let him remember Lyell's profound remark, 
that the thickness and extent of sedimentary formations 


are the result and measure of the degradation which 
the earth's crust has elsewhere suffered. And what an 
amount of degradation is implied by the sedimentary 
deposits of many countries! Professor Eamsay has 
given me the maximum thickness, in most cases from 
actual measurement, in a few cases from estimate, of 
each formation in different parts of Great Britain ; and 
this is the result : — 

Palaeozoic strata (not including igneous beds) .. 57,154 

Secondary strata 13,190 

Tertiary strata 2,240 

— making altogether 72,584 feet ; that is, very nearly 
thirteen and three-quarters British miles. Some of 
these formations, which are represented in England 
by thin beds, are thousands of feet in thickness on the 
Continent. Moreover, between each successive formation, 
we have, in the opinion of most geologists, enormously 
long blank periods. So that the lofty pile of sedimen- 
tary rocks in Britain, gives but an inadequate idea of 
the time which has elapsed during their accumulation ; 
yet what time this must have consumed! Good ob- 
servers have estimated that sediment is deposited by the 
great Mississippi river at the rate of only GOO feet in a 
hundred thousand years. Tins estimate may be quite 
erroneous ; yet, considering over what wide spaces very 
fine sediment is transported by the currents of the 
sea, the process of accumulation in any one area must 
be extremely slow. 

But the amount of denudation which the strata have 
in many places suffered, independently of the rate of 
accumulation of the degraded matter, probably offers 
the best evidence of the lapse of time. I remember 
having been much struck with the evidence of dei nida- 
tion, when viewing volcanic islands, which have been 


worn by the waves and pared all round into perpendicular 
cliffs of one or two thousand feet in height; for the 
gentle slope of the lava-streams, due to their formerly 
liquid state, showed at a glance how far the hard, rocky 
beds had once extended into the open ocean. The 
same story is still more plainly told by faults, — those 
great cracks along which the strata have been upheaved 
on one side, or thrown down on the other, to the 
height or depth of thousands of feet ; for since the crust 
cracked, the surface of the land has been so completely 
planed down by the action of the sea, that no trace of 
these vast dislocations is externally visible. 

The Craven fault, for instance, extends for upwards 
of 30 miles, and along this line the vertical displace- 
ment of the strata has varied from 600 to 3000 feet. 
Prof. Kamsay has published an account of a downthrow 
in Anglesea of 2300 feet ; and he informs me that he 
fully believes there is one in Merionethshire of 12,000 
feet ; yet in these cases there is nothing on the surface 
to show such prodigious movements ; the pile of rocks 
on the one or other side having been smoothly swept 
away. The consideration of these facts impresses my 
mind almost in the same manner as does the vain en- 
deavour to grapple with the idea of eternity. 

I am tempted to give one other case, the well-known 
one of the denudation of the Weald. Though it must 
be admitted that the denudation of the Weald has been 
a mere trifle, in comparison with that which has 
removed masses of our palaeozoic strata, in parts ten 
thousand feet in tliickness, as shown in Prof. Kamsay's 
masterly memoir on tins subject. Yet it is an admirable 
lesson to stand on the North Downs and to look at the 
distant South Downs; for, remembering that at no 
great distance to the west the northern and southern 
escarpments meet and close, one can safely picture to 


oneself the great dome of rocks which must have covered 
up the Weald within so limited a period as since the 
latter part of .the Chalk formation. The distance from 
the northern to the southern Downs is about 22 miles, 
and the thickness of the several formations is on an ave- 
rage about 1100 feet, as I am informed by Prof. Eamsay. 
But if, as some geologists suppose, a range of older rocks 
underlies the Weald, on the flanks of which the over- 
lying sedimentary deposits might have accumulated in 
thinner masses than elsewhere, the above estimate would 
be erroneous ; but this source of doubt probably would 
not greatly affect the estimate as applied to the western 
extremity of the district. If, then, we knew the rate at 
which the sea commonly wears away a line of cliff of 
any given height, we could measure the time requisite 
to have denuded the Weald. This, of course, cannot 
be done ; but we may, in order to form some crude 
notion on the subject, assume that the sea would eat 
into cliffs 500 feet in height at the rate of one inch 
in a century. This will at first appear much too 
small an allowance ; but it is the same as if we were to 
assume a cliff one yard in height to be eaten back 
along a whole line of coast at the rate of one yard in 
nearly every twenty-two years. I doubt whether any 
rock, even as soft as chalk, would yield at this rate ex- 
cepting on the most exposed coasts ; though no doubt the 
degradation of a lofty cliff would be more rapid from the 
breakage of the fallen fragments. On the other hand, 
I do not believe that any line of coast, ten or twenty 
miles in length, ever suffers degradation at the same time 
along its whole indented length ; and we must remember 
that almost all strata contain harder layers or nodules, 
which from long resisting attrition form a breakwater 
at the base. Hence, under ordinary circumstances, I 
conclude that for a cliff 500 feet in height, a denudation 


of one inch per century for the whole length would be 
an ample allowance. At this rate, on the above 
data, the denudation of the Weald must have required 
306,662,400 years ; or say three hundred million years. 

The action of fresh water on the gently inclined 
Wealden district, when upraised, could hardly have 
been great, but it would somewhat reduce the above 
estimate. On the other hand, during oscillations of 
level, which we know this area has undergone, the sur- 
face may have existed for millions of years as land, and 
thus have escaped the action of the sea : when deeply 
submerged for perhaps equally long periods, it would, 
likewise, have escaped the action of the coast-waves. 
So that in all probability a far longer period than 300 
million years has elapsed since the latter part of the 
Secondary period. 

I have made these few remarks because it is highly 
important for us to gain some notion, however imper- 
fect, of the lapse of years. During each of these years, 
over the whole world, the land and the water has been 
peopled by hosts of living forms. What an infinite 
number of generations, which the mind cannot grasp, 
must have succeeded each other in the long roll of 
years! Now turn to our richest geological museums, 
and what a paltry display we behold ! 

On the poorness of our Palceontological collections. — 
That our palseontological collections are very imperfect, 
is admitted by eveiy one. The remark of that admir- 
able palaeontologist, the late Edward Forbes, should not 
be forgotten, namely, that numbers of our fossil species 
are known and named from single and often broken 
specimens, or from a few specimens collected on some 
one spot. Only a small portion of the surface of the 
earth has been geologically explored, and no part with 


sufficient care, as the important discoveries made every 
year in Europe prove. No organism wholly soft can be 
preserved. Shells and bones will decay and disappear 
when left on the bottom of the sea, where sediment is 
not accumulating. I believe we are continually taking 
a most erroneous view, when we tacitly admit to our- 
selves that sediment is being deposited over nearly the 
whole bed of the sea, at a rate sufficiently quick to 
embed and preserve fossil remains. Throughout an 
enormously large proportion of the ocean, the bright 
blue tint of the water bespeaks its purity. The many 
cases on record of a formation conformably covered, 
after an enormous interval of time, by another and 
later formation, without the underlying bed having 
suffered in the interval any wear and tear, seem ex- 
plicable only on the view of the bottom of the sea not 
rarely lying for ages in an unaltered condition. The 
remains which do become embedded, if in sand or gravel, 
will when the beds are upraised generally be dissolved 
by the percolation of rain-water. I suspect that but few 
of the very many animals which live on the beach be- 
tween high and low watermark are preserved. For in- 
stance, the several species of the Chthamalmse (a sub- 
family of sessile cirripedes) coat the rocks all over the 
world in infinite numbers: they are all strictly littoral, 
with the exception of a single Mediterranean species, 
which inhabits deep water and has been found fossil in 
Sicily, whereas not one other species has hitherto been 
found in any tertiary formation : yet it is now known 
that the genus Chthamalus existed during the chalk 
period. The molluscan genus Chiton offers a partially 
analogous case. 

With respect to the terrestrial productions which 
lived during the Secondary and Palaeozoic periods, it is 
superfluous to state that our evidence from fossil 


remains is fragmentary in an extreme degree. For 
instance, not a land shell is known belonging to 
either of these vast periods, with one exception disco- 
vered by Sir C. Lyell in the carboniferous strata of 
North America. In regard to mammiferous remains, a 
single glance at the historical table published in the 
Supplement to Lyell's Manual, will bring home the 
truth, how accidental and rare is their preservation, far 
better than pages of detail. Nor is their rarity sur- 
prising, when we remember how large a proportion of 
the bones of tertiary mammals have been discovered 
either in caves or in lacustrine deposits ; and that not a 
cave or true lacustrine bed is known belonging to the 
age of our secondary or palaeozoic formations. 

But the imperfection in the geological record mainly 
results from another and more important cause than any 
of the foregoing; namely, from the several formations 
being separated from each other by wide intervals of 
time. When we see the formations tabulated in written 
works, or when we follow them in nature, it is 
difficult to avoid believing that they are closely con- 
secutive. But we know, for instance, from Sir K. 
Murchison's great work on Kussia, what wide gaps 
there are in that country between the superimposed 
formations; so it is in North America, and in many 
other parts of the world. The most skilful geologist, if 
his attention had been exclusively confined to these 
large territories, would never have suspected that during 
the periods which were blank and barren in his own 
country, great piles of sediment, charged with new and 
peculiar forms of life, had elsewhere been accumu- 
lated. And if in each separate territory, hardly any 
idea can be formed of the length of time which has 
elapsed between the consecutive formations, we may infer 
that this could nowhere be ascertained. The frequent 



and great changes in the mineralogical composition of 
consecutive formations, generally implying great changes 
in the geography of the surrounding lands, whence the 
sediment has been derived, accords with the belief of 
vast intervals of time having elapsed between each for- 

But we can, I think, see why the geological forma- 
tions of each region are almost invariably inter- 
mittent ; that is, have not followed each other in close 
sequence. Scarcely any fact struck me more when 
examining many hundred miles of the South American 
coasts, which have been upraised several hundred feet 
within the recent period, than the absence of any recent 
deposits sufficiently extensive to last for even a short 
geological period. Along the whole west coast, which 
is inhabited by a peculiar marine fauna, tertiary beds 
are so scantily developed, that no record of several suc- 
cessive and peculiar marine faunas will probably be 
preserved to a distant age. A little reflection will ex- 
plain why along the rising coast of the western side of 
South America, no extensive formations with recent or 
tertiary remains can anywhere be found, though the 
supply of sediment must for ages have been great, from 
the enormous degradation of the coast-rocks and from 
muddy streams entering the sea. The explanation, no 
doubt, is, that the littoral and sub-littoral deposits are 
continually worn away, as soon as they are brought up 
by the slow and gradual rising of the land within the 
grinding action of the coast- waves. 

We may, I think, safely conclude that sediment must 
be accumulated in extremely thick, solid, or extensive 
masses, in order to withstand the incessant action of 
the waves, when first upraised and during subsequent 
oscillations of level. Such thick and extensive accumu- 
lations of sediment may be formed in two ways ; either, 


in profound depths of the sea, in which case, judging 
from the researches of E. Forbes, we may conclude that 
the bottom will be inhabited by extremely few animals, 
and the mass when upraised will give a most imperfect 
record of the forms of life which then existed ; or, sedi- 
ment may be accumulated to any thickness and extent 
over a shallow bottom, if it continue slowly to subside. 
In this latter case, as long as the rate of subsidence 
and supply of sediment nearly balance each other, 
the sea will remain shallow and favourable for life, and 
thus a fossiliferous formation thick enough, when up- 
raised, to resist any amount of degradation, may be 

I am convinced that all our ancient formations, 
which are rich in fossils, have thus been formed 
during subsidence. Since publishing my views on 
this subject in 1845, I have watched the progress of 
Geology, and have been surprised to note how author 
after author, in treating of this or that great formation, 
has come to the conclusion that it was accumulated 
during subsidence. I may add, that the only ancient 
tertiary formation on the west coast of South America, 
which has been bulky enough to resist such degradation 
as it has as yet suffered, but which will hardly last to a 
distant geological age, was certainly deposited during a 
downward oscillation of level, and thus gained consi- 
derable thickness. 

All geological facts tell us plainly that each area 
has undergone numerous slow oscillations of level, and 
apparently these oscillations have affected wide spaces. 
Consequently formations rich in fossils and sufficiently 
thick and extensive to resist subsequent degradation, 
may have been formed over wide spaces during periods 
of subsidence, but only where the supply of sediment 
was sufficient to keep the sea shallow and to embed and 

o 2 


preserve the remains before they had time to decay. 
On the other hand, as long as the bed of the sea re- 
mained stationary, thick deposits conld not have been 
accumulated in the shallow parts, which are the most 
favourable to life. Still less could this have happened 
during the alternate periods of elevation ; or, to speak 
more accurately, the beds which were then accumu- 
lated will have been destroyed by being upraised and 
brought within the limits of the coast-action. 

Thus the geological record will almost necessarily be 
rendered intermittent. I feel much confidence in the 
truth of these views, for they are in strict accordance with 
the general principles inculcated by Sir C. Lyell ; and 
E. Forbes independently arrived at a similar conclusion. 

One remark is here worth a passing notice. During 
periods of elevation the area of the land and of the 
adjoining shoal parts of the sea will be increased, and 
new stations will often be formed ; — all circumstances 
most favourable, as previously explained, for the form- 
ation of new varieties and species ; but during such 
periods there will generally be a blank in the geological 
record. On the other hand, during subsidence, the 
inhabited area and number of inhabitants will decrease 
(excepting the productions on the shores of a continent 
when first broken up into an archipelago), and conse- 
quently during subsidence, though there will be much 
extinction, fewer new varieties or species will be formed ; 
and it is during these very periods of subsidence, that 
our great deposits rich in fossils have been accumulated. 
Nature may almost be said to have guarded against the 
frequent discovery of her transitional or linking forms. 

From the foregoing considerations it cannot be doubted 
that the geological record, viewed as a whole, is ex- 
tremely imperfect; but if we confine our attention to 
any one formation, it becomes more difficult to under- 


stand, why we do not therein find closely graduated 
varieties between the allied species which lived at its 
commencement and at its close. Some cases are on 
record of the same species presenting distinct varieties 
in the upper and lower parts of the same formation, but, 
as they are rare, they may be here passed over. Al- 
though each formation has indisputably required a 
vast number of years for its deposition, I can see several 
reasons why each should not include a graduated series 
of links between the species which then lived ; but I can 
by no means pretend to assign due proportional weight 
to the following considerations. 

Although each formation may mark a very long lapse 
of years, each perhaps is short compared with the period 
requisite to change one species into another. I am 
aware that two palaeontologists, whose opinions are 
worthy of much deference, namely Bronn and Wood- 
ward, have concluded that the average duration of each 
formation is twice or thrice as long as the average 
duration of specific forms. But insuperable difficulties, 
as it seems to me, prevent us coming to any just con- 
clusion on this head. When we see a species first ap- 
pearing in the middle of any formation, it would be rash 
in the extreme to infer that it had not elsewhere pre- 
viously existed. So again when we find a species disap- 
pearing before the uppermost layers have been deposited, 
it would be equally rash to suppose that it then became 
wholly extinct. We forget how small the area of Eu- 
rope is compared with the rest of the world ; nor have 
the several stages of the same formation throughout 
Europe been correlated with perfect accuracy. 

With marine animals of all kinds, we may safely 
infer a large amount of migration during climatal 
and other changes; and when we see a species first 
appearing in any formation, the probability is that it 



I only then first immigrated into that area. It is well 
) known, for instance, that several species appeared some- 
what earlier in the palaeozoic beds of North America 
than in those of Europe ; time having apparently been 
required for their migration from the American to the 
European seas. In examining the latest deposits of 
various quarters of the world, it has everywhere been 
noted, that some few still existing species are common 
in the deposit, but have become extinct in the immedi- 
ately surrounding sea ; or, conversely, that some are 
now abundant in the neighbouring sea, but are rare or 
absent in this particular deposit. It is an excellent 
lesson to reflect on the ascertained amount of migration 
of the inhabitants of Europe during the Glacial period, 
which forms only a part of one whole geological period ; 
and likewise to reflect on the great changes of level, 
on the inordinately great change of climate, on the 
prodigious lapse of time, all included within this same 
glacial period. Yet it may be doubted whether in any 
quarter of the world, sedimentary deposits, including 
fossil remains, have gone on accumulating within the 
same area during the whole of this period. It is not, 
for instance, probable that sediment was deposited dur- 
ing the whole of the glacial period near the mouth of 
the Mississippi, within that limit of depth at which ma- 
rine animals can flourish ; for we know what vast geo- 
graphical changes occurred in other parts of America 
during this space of time. When such beds as were 
deposited in shallow water near the mouth of the Mis- 
sissippi during some part of the glacial period shall have 
been upraised, organic remains will probably first appear 
and disappear at different levels, owing to the migration 
of species and to geographical changes. And in the 
distant future, a geologist examining these beds, might 
be tempted to conclude that the average duration of life 


of the embedded fossils had been less than that of the 
glacial period, instead of having been really far greater, 
that is extending from before the glacial epoch to the 
present day. 

In order to get a perfect gradation between two forms 
in the upper and lower parts of the same formation, the 
deposit must have gone on accumulating for a very long 
period, in order to have given sufficient time for the 
slow process of variation ; hence the deposit will gene- 
rally have to be a very thick one ; and the species un- 
dergoing modification will have had to live on the same 
area throughout this whole time. But we have seen 
that a thick fossiliferous formation can only be accumu- 
lated during a period of subsidence ; and to keep the 
depth approximately the same, which is necessary in 
order to enable the same species to live on the same 
space, the supply of sediment must nearly have counter- 
balanced the amount of subsidence. But this same 
movement of subsidence will often tend to sink the 
area whence the sediment is derived, and thus diminish 
the supply whilst the downward movement continues. 
In fact, this nearly exact balancing between the supply 
of sediment and the amount of subsidence is probably 
a rare contingency ; for it has been observed by more 
than one palaeontologist, that very thick deposits are 
usually barren of organic remains, except near their 
upper or lower limits. 

It would seem that each separate formation, like the 
whole pile of formations in any country, has generally 
been intermittent in its accumulation. When we see, 
as is so often the case, a formation composed of beds 
of different mineralogical composition, we may reason- 
ably suspect that the process of deposition has been 
much interrupted, as a change in the currents of the 
sea and a supply of sediment of a different nature will 


generally have been due to geographical changes re- 
quiring much time. Nor will the closest inspection of 
a formation give any idea of the time which its depo- 
sition has consumed. Many instances could be given of 
beds only a few feet in thickness, representing forma- 
tions, elsewhere thousands of feet in thickness, and 
which must have required an enormous period for their 
accumulation ; yet no one ignorant of this fact would 
have suspected the vast lapse of time represented by 
the thinner formation. Many cases could be given of 
the lower beds of a formation having been upraised, 
denuded, submerged, and then re-covered by the upper 
beds of the same formation, — facts, showing what wide, 
yet easily overlooked, intervals have occurred in its accu- 
mulation. In other cases we have the plainest evidence 
in great fossilised trees, still standing upright as they 
grew, of many long intervals of time and changes of 
level during the process of deposition, winch would 
never even have been suspected, had not the trees 
chanced to have been preserved: thus, Messrs. Lyell 
and Dawson found carboniferous beds 1400 feet thick 
in Nova Scotia, with ancient root-bearing strata, one 
above the other, at no less than sixty-eight different 
levels. Hence, when the same species occur at the 
bottom, middle, and top of a formation, the probability 
is that they have not lived on the same spot during the 
whole period of deposition, but have disappeared and 
reappeared, perhaps many times, during the same geo- 
logical period. So that if such species were to undergo 
a considerable amount of modification during any one 
geological period, a section would not probably include 
all the fine intermediate gradations which must on my 
theory have existed between them, but abrupt, though 
perhaps very slight, changes of form. 

It is all-important to remember that naturalists have 


no golden rule by which to distinguish species and va- 
rieties ; they grant some little variability to each species, 
but when they meet with a somewhat greater amount 
of difference between any two forms, they rank both as 
species, unless they are enabled to connect them to- 
gether by close intermediate gradations. And this from 
the reasons just assigned we can seldom hope to effect 
in any one geological section. Supposing B and C to 
be two species, and a third, A, to be found in an 
underlying bed ; even if A were strictly intermediate 
between B and C, it would simply be ranked as a third 
and distinct species, unless at the same time it could be 
most closely connected with either one or both forms by 
intermediate varieties. Nor should it be forgotten, as 
before explained, that A might be the actual progenitor 
of B and C, and yet might not at all necessarily be 
strictly intermediate between them in all points of struc- 
ture. So that we might obtain the parent-species and 
its several modified descendants from the lower and 
upper beds of a formation, and unless we obtained 
numerous transitional gradations, we should not recog- 
nise their relationship, and should consequently be com- 
pelled to rank them all as distinct species. 

It is notorious on what excessively slight differences 
many palaeontologists have founded their species; and 
they do this the more readily if the specimens come 
from different sub-stages of the same formation. Some 
experienced conchologists are now sinking many of the 
very fine species of D'Orbigny and others into the rank 
of varieties ; and on this view we do find the kind of 
evidence of change which on my theory we ought to 
find. Moreover, if we look to rather wider intervals, 
namely, to distinct but consecutive stages of the same 
great formation, we find that the embedded fossils, 
though almost universally ranked as specifically different, 

o 3 


yet are far more closely allied to each other than 
are the species found in more widely separated forma- 
tions ; but to this subject I shall have to return in the 
following chapter. 

One other consideration is worth notice : with animals 
and plants that can propagate rapidly and are not 
highly locomotive, there is reason to suspect, as we 
have formerly seen, that their varieties are generally at 
first local ; and that such local varieties do not spread 
widely and supplant their parent-forms until they have 
been modified and perfected in some considerable de- 
gree. According to this view, the chance of discovering 
in a formation in any one country all the early stages 
of transition between any two forms, is small, for the 
successive changes are supposed to have been local or 
confined to some one spot. Most marine animals have 
a wide range ; and we have seen that with plants it is 
those which have the widest range, that oftenest present 
varieties ; so that with shells and other marine animals, 
it is probably those which have had the widest range, 
far exceeding the limits of the known geological forma- 
tions of Europe, which have oftenest given rise, first 
to local varieties and ultimately to new species ; and 
this again would greatly lessen the chance of our being 
able to trace the stages of transition in any one geo- 
logical formation. 

It should not be forgotten, that at the present day, 
with perfect specimens for examination, two forms can 
seldom be connected by intermediate varieties and thus 
proved to be the same species, until many specimens 
have been collected from many places ; and in the case 
of fossil species this could rarely be effected by palae- 
ontologists. We shall, perhaps, best perceive the im- 
probability of our being enabled to connect species by 
numerous, fine, intermediate, fossil links, by asking 


ourselves whether, for instance, geologists at some 
future period will be able to prove, that our different 
breeds of cattle, sheep, horses, and dogs have descended 
from a single stock or from several aboriginal stocks ; 
or, again, whether certain sea-shells inhabiting the 
shores of North America, which are ranked by some con- 
chologists as distinct species from their European repre- 
sentatives, and by other conchologists as only varieties, 
are really varieties or are, as it is called, specifically 
distinct. This could be effected only by the future geo- 
logist discovering in a fossil state numerous intermediate 
gradations ; and such success seems to me improbable in 
the highest degree. 

Geological research, though it has added numerous 
species to existing and extinct genera, and has made the 
intervals between some few groups less wide than they 
otherwise would have been, yet has done scarcely any- 
thing in breaking down the distinction between species, 
by connecting them together by numerous, fine, inter- 
mediate varieties ; and this not having been effected, 
is probably the gravest and most obvious of all the 
many objections which may be urged against my views. 
Hence it will be worth while to sum up the foregoing 
remarks, under an imaginary illustration. The Malay 
Archipelago is of about the size of Europe from the 
North Cape to the Mediterranean, and from Britain to 
Russia ; and therefore equals all the geological forma- 
tions which have been examined with any accuracy, 
excepting those of the United States of America. 
I fully agree with Mr. Godwin-Austen, that the 
present condition of the Malay Archipelago, with its 
numerous large islands separated by wide and shallow 
seas, probably represents the former state of Europe, 
when most of our formations were accumulating. The 
Malay Archipelago is one of the richest regions of the 


whole world in organic beings; yet if all the species 
were to be collected which have ever lived there, how 
imperfectly would they represent the natural history of 
the world ! 

But we have every reason to believe that the terres- 
trial productions of the archipelago would be preserved 
in an excessively imperfect manner in the formations 
which we suppose to be there accumulating. I suspect 
that not many of the strictly littoral animals, or of 
those which lived on naked submarine rocks, would 
be embedded ; and those embedded in gravel or sand, 
would not endure to a distant epoch. Wherever sedi- 
ment did not accumulate on the bed of the sea, or where 
it did not accumulate at a sufficient rate to protect 
organic bodies from decay, no remains could be pre- 

In our archipelago, I believe that fossiliferous forma- 
tions could be formed of sufficient thickness to last to 
an age, as distant in futurity as the secondary forma- 
tions lie in the past, only during periods of subsidence. 
These periods of subsidence would be separated from 
each other by enormous intervals, during which the 
area would be either stationary or rising ; whilst rising, 
each fossiliferous formation would be destroyed, almost 
as soon as accumulated, by the incessant coast-action, as 
we now see on the shores of South America. During 
the periods of subsidence there would probably be much 
extinction of life ; during the periods of elevation, there 
would be much variation, but the geological record 
would then be least perfect. 

It may be doubted whether the duration of any one 
great period of subsidence over the whole or part of 
the archipelago, together with a contemporaneous accu- 
mulation of sediment, would exceed the average duration 
of the same specific forms ; and these contingencies are 


indispensable for the preservation of all the transitional 
gradations between any two or more species. If such 
gradations were not fully preserved, transitional varieties 
would merely appear as so many distinct species. It is, 
also, probable that each great period of subsidence would 
be interrupted by oscillations of level, and that slight 
climatal changes would intervene during such lengthy 
periods ; and in these cases the inhabitants of the archi- 
pelago would have to migrate, and no closely consecu- 
tive record of their modifications could be preserved in 
any one formation. 

Very many of the marine inhabitants of the archipe- 
lago now range thousands of miles beyond its confines ; 
and analogy leads me to believe that it would be chiefly 
these far-ranging species which would oftenest produce 
new varieties; and the varieties would at first gene- 
rally be local or confined to one place, but if possessed 
of any decided advantage, or when further modified and 
improved, they would slowly spread and supplant their 
parent-forms. When such varieties returned to their 
ancient homes, as they would differ from their former 
state, in a nearly uniform, though perhaps extremely 
slight degree, they would, according to the principles 
followed by many palaeontologists, be ranked as new and 
distinct species. 

If then, there be some degree of truth in these 
remarks, we have no right to expect to find in our 
geological formations, an infinite number of those fine 
transitional forms, which on my theory assuredly have 
connected all the past and present species of the same 
group into one long and branching chain of life. We 
ought only to look for a few links, some more closely, 
some more distantly related to each other ; and these 
links, let them be ever so close, if found in different 
stages of the same formation, would, by most palaeonto- 


logists, be ranked as distinct species. But I do not pre- 
tend that I should ever have suspected how poor a 
record of the mutations of life, the best preserved 
geological section presented, had not the difficulty of 
our not discovering innumerable transitional links 
between the species which appeared at the commence- 
ment and close of each formation, pressed so hardly 
on my theory. 

On the sudden appearance of whole groups of Allied 
Species. — The abrupt manner in which whole groups of 
species suddenly appear in certain formations, has been 
urged by several palaeontologists, for instance, by 
Agassiz, Pictet, and by none more forcibly than by 
Professor Sedgwick, as a fatal objection to the belief in 
the transmutation of species. If numerous species, 
belonging to the same genera or families, have really 
started into life all at once, the fact would be fatal to 
the theory of descent with slow modification through 
natural selection. For the development of a group 
of forms, all of which have descended from some 
one progenitor, must have been an extremely slow 
process ; and the progenitors must have lived long ages 
before their modified descendants. But we continually 
over-rate the perfection of the geological record, and 
falsely infer, because certain genera or families have 
not been found beneath a certain stage, that they did 
not exist before that stage. We continually forget 
how large the world is, compared with the area over 
which our geological formations have been carefully 
examined ; we forget that groups of species may else- 
where have long existed and have slowly multiplied 
before they invaded the ancient archipelagoes of Europe 
and of the United States. We do not make due allow- 
ance for the enormous intervals of time, which have 


probably elapsed between our consecutive formations, — 
longer perhaps in some cases than the time required 
for the accumulation of each formation. These intervals 
will have given time for the multiplication of species 
from some one or some few parent-forms ; and in the 
succeeding formation such species will appear as if sud- 
denly created. 

I may here recall a remark formerly made, namely 
that it might require a long succession of ages to adapt 
an organism to some new and peculiar line of life, for 
instance to fly through the air ; but that when this had 
been effected, and a few species had thus acquired a 
great advantage over other organisms, a comparatively 
short time would be necessary to produce many di- 
vergent forms, which would be able to spread rapidly 
and widely throughout the world. 

I will now give a few examples to illustrate these 
remarks ; and to show how liable we are to error in 
supposing that whole groups of species have suddenly 
been produced. I may recall the well-known fact that 
in geological treatises, published not many years ago, 
the great class of mammals was always spoken of as 
having abruptly come in at the commencement of the 
tertiary series. And now one of the richest known 
accumulations of fossil mammals belongs to the middle 
of the secondary series ; and one true mammal has 
been discovered in the new red sandstone at nearly the 
commencement of tins great series. Cuvier used to 
urge that no monkey occurred in any tertiary stratum ; 
but now extinct species have . been discovered in India, 
South America, and in Europe even as far back as the 
eocene stage. The most striking case, however, is that 
of the Whale family ; as these animals have huge bones, 
are marine, and range over the world, the fact of not 
a single bone of a whale having been discovered in 


any secondary formation, seemed fully to justify the 
belief that this great and distinct order had been sud- 
denly produced in the interval between the latest 
secondary and earliest tertiary formation. But now we 
may read in the Supplement to Lyell's ' Manual/ pub- 
lished in 1858, clear evidence of the existence of whales 
in the upper greensand, some time before the close of 
the secondary period. 

I may give another instance, which from having 
passed under my own eyes has much struck me. In a 
memoir on Fossil Sessile Cirripedes, I have stated that, 
from the number of existing and extinct tertiary species ; 
from the extraordinary abundance of the individuals of 
many species all over the world, from the Arctic regions 
to the equator, inhabiting various zones of depths from 
the upper tidal limits to 50 fathoms ; from the perfect 
manner in which specimens are preserved in the oldest 
tertiary beds ; from the ease with which even a frag- 
ment of a valve can be recognised ; from all these cir- 
cumstances, I inferred that had sessile cirripedes existed 
during the secondary periods, they would certainly have 
been preserved and discovered ; and as not one species 
had been discovered in beds of this age, I concluded 
that this great group had been suddenly developed at 
the commencement of the tertiary series. This was a 
sore trouble to me, adding as I thought one more in- 
stance of the abrupt appearance of a great group of 
species. But my work had hardly been published, when 
a skilful palaeontologist, M. Bosquet, sent me a drawing 
of a perfect specimen of an unmistakeable sessile cirri- 
pede, which he had himself extracted from the chalk 
of Belgium. And, as if to make the case as striking as 
possible, this sessile cirripede was a Chthamalus, a very 
common, large, and ubiquitous genus, of which not one 
specimen has as yet been found even in any tertiary 


stratum. Hence we now positively know that sessile 
cirripedes existed during the secondary period; and 
these cirripedes might have been the progenitors of our 
many tertiary and existing species. 

The case most frequently insisted on by palaeonto- 
logists of the apparently sudden appearance of a whole 
group of species, is that of the teleostean fishes, low 
down in the Chalk period. This group includes the 
large majority of existing species. Lately, Professor 
Pictet has carried their existence one sub-stage further 
back; and some palaeontologists believe that certain 
much older fishes, of which the affinities are as yet im- 
perfectly known, are really teleostean. Assuming, how- 
ever, that the whole of them did appear, as Agassiz 
believes, at the commencement of the chalk formation, 
the fact would certainly be highly remarkable ; but I 
cannot see that it would be an insuperable difficulty on 
my theory, unless it could likewise be shown that the 
species of this group appeared suddenly and simul- 
taneously throughout the world at this same period. It 
is almost superfluous to remark that hardly any fossil- 
fish are known from south of the equator; and by 
running through Pictet's Palaeontology it will be seen 
that very few species are known from several formations 
in Europe. Some few families of fish now have a 
confined range; the teleostean fish might formerly 
have had a similarly confined range, and after having 
been largely developed in some one sea, might have 
spread widely. Nor have we any right to suppose that 
the seas of the world have always been so freely open 
from south to north as they are at present. Even at 
this day, if the Malay Archipelago were converted into 
land, the tropical parts of the Indian Ocean would form 
a large and perfectly enclosed basin, in which any great 
group of marine animals might be multiplied; and 


here they would remain confined, until some of the 
species became adapted to a cooler climate, and were 
enabled to double the southern capes of Africa or 
Australia, and thus reach other and distant seas. 

From these and similar considerations, but chiefly 
from our ignorance of the geology of other countries 
beyond the confines of Europe and the United States ; 
and from the revolution in our palaeontological ideas 
on many points, which the discoveries of even the last 
dozen years have effected, it seems to me to be about 
as rash in us to dogmatize on the succession of organic 
beings throughout the world, as it would be for a 
naturalist to land for five minutes on some one barren 
point in Australia, and then to discuss the number 
and range of its productions. 

On the sudden appearance of groups of Allied Species in 
the lowest known fossiliferous strata. — There is another 
and allied difficulty, which is much graver. I allude 
to the manner in which numbers of species of the same 
group, suddenly appear in the lowest known fossiliferous 
rocks. Most of the arguments which have convinced 
me that all the existing species of the same group hare 
descended from one progenitor, apply with nearly equal 
force to the earliest known species. For instance, I 
cannot doubt that all the Silurian trilobites have 
descended from some one crustacean, which must have 
lived long before the Silurian age, and which probably 
differed greatly from any known animal. Some of 
the most ancient Silurian animals, as the Nautilus, 
Lingula, &c, do not differ much from living species ; 
and it cannot on my theoiy be supposed, that these old 
species were the progenitors of all the species of the 
orders to which they belong, for they do not present 
characters in any degree intermediate between them. 


If, moreover, they had been the progenitors of these 
orders, they would almost certainly have been long ago 
supplanted and exterminated by their numerous and 
improved descendants. 

Consequently, if my theory be true, it is indisputable 
that before the lowest Silurian stratum was deposited, 
long periods elapsed, as long as, or probably far longer 
than, the whole interval from the Silurian age to the 
present day ; and that during these vast, yet quite un- 
known, periods of time, the world swarmed with living 

To the question why we do not find records of these 
vast primordial periods, I can give no satisfactory 
answer. Several of the most eminent geologists, with 
Sir K. Murchison at their head, are convinced that we 
see in the organic remains of the lowest Silurian stratum 
the dawn of life on this planet. Other highly com- 
petent judges, as Lyell and the late E. Forbes, dispute 
this conclusion. We should not forget that only a small 
portion of the world is known with accuracy. M. Bar- 
rande has lately added another and lower stage to the 
Silurian system, abounding with new and peculiar 
species. Traces of life have been detected in the Long- 
mynd beds beneath Barrande's so-called primordial 
zone. The presence of phosphatic nodules and bitu- 
minous matter in some of the lowest azoic rocks, pro- 
bably indicates the former existence of life at these 
periods. But the difficulty of understanding the ab- 
sence of vast piles of fossiliferous strata, which on my 
theory no doubt were somewhere accumulated before 
the Silurian epoch, is very great. If these most ancient 
beds had been wholly worn away by denudation, or 
obliterated by metamorphic action, we ought to find 
only small remnants of the formations next succeeding 
them in age, and these ought to be very generally in 


a metamorphosed condition. But the descriptions which 
we now possess of the Silurian deposits over immense 
territories in Kussia and in North America, do not sup- 
port the view, that the older a formation is, the more it 
has suffered the extremity of denudation and metamor- 

The case at present must remain inexplicable ; and 
may be truly urged as a valid argument against the 
views here entertained. To show that it may hereafter 
receive some explanation, I will give the following 
hypothesis. From the nature of the organic remains, 
which do not appear to have inhabited profound 
depths, in the several formations of Europe and of the 
United States ; and from the amount of sediment, miles 
in thickness, of which the formations are composed, we 
may infer that from first to last large islands or tracts 
of land, whence the sediment was derived, occurred in 
the neighbourhood of the existing continents of Europe 
and North America. But we do not know what was 
the state of things in the intervals between the suc- 
cessive formations ; whether Europe and the United 
States during these intervals existed as dry land, or 
as a submarine surface near land, on which sediment 
was not deposited, or again as the bed of an open and 
unfathomable sea. 

Looking to the existing oceans, which are thrice as 
extensive as the land, we see them studded with many 
islands; but not one oceanic island is as yet known 
to afford even a remnant of any palaeozoic or secondary 
formation. Hence we may perhaps infer, that during 
the palaeozoic and secondary periods, neither continents 
nor continental islands existed where our oceans now 
extend; for had they existed there, palaeozoic and 
secondary formations would in all probability have been 
accumulated from sediment derived from their wear and 


tear ; and would have been at least partially upheaved 
by the oscillations of level, which we may fairly con- 
clude must have intervened during these enormously 
long periods. If then we may infer anything from 
these facts, we may infer that where our oceans now 
extend, oceans have extended from the remotest period 
of which we have any record ; and on the other hand, 
that where continents now exist, large tracts of land 
have existed, subjected no doubt to great oscillations of 
level, since the earliest silurian period. The coloured 
map appended to my volume on Coral Keefs, led me to 
conclude that the great oceans are still mainly areas of 
subsidence, the great archipelagoes still areas of oscilla- 
tions of level, and the continents areas of elevation. 
But have we any right to assume that things have thus 
remained from eternity ? Our continents seem to have 
been formed by a preponderance, during many oscilla- 
tions of level, of the force of elevation ; but may not the 
areas of preponderant movement have changed in the 
lapse of ages ? At a period immeasurably antecedent to 
the silurian epoch, continents may have existed where 
oceans are now spread out ; and clear and open oceans 
may have existed where our continents now stand. Nor 
should we be justified in assuming that if, for instance, 
the bed of the Pacific Ocean were now converted into 
a continent, we should there find formations older 
than the silurian strata, supposing such to have been 
formerly deposited ; for it might well happen that strata 
which had subsided some miles nearer to the centre of 
the earth, and which had been pressed on by an enor- 
mous weight of superincumbent water, might have 
undergone far more metamorphic action than strata 
which have always remained nearer to the surface. The 
immense areas in some parts of the world, for instance 
in South America, of bare metamorphic rocks, which 


must have been heated under great pressure, have 
always seemed to me to require some special explana- 
tion ; and we may perhaps believe that we see in these 
large areas, the many formations long anterior to the 
silurian epoch in a completely metamorphosed con- 

The several difficulties hp^o discussed, namely our not 
finding in the successive formations infinitely numerous 
transitional links between the many species which now 
exist or have existed; the sudden manner in which 
whole groups of species appear in our European forma- 
tions ; the almost entire absence, as at present known, 
of fossiliferous formations beneath the Silurian strata, 
are all undoubtedly of the gravest nature. We see this 
in the plainest manner by the fact that all the most 
eminent paleontologists, namely Cuvier, Owen, Agassiz, 
Barrande, Falconer, E. Forbes, &c, and all our greatest 
geologists, as Lyell, Murchison, Sedgwick, &c, have 
unanimously, often vehemently, maintained the immuta- 
bility of species. But I have reason to believe that one 
great authority, Sir Charles Lyell, from further re- 
flexion entertains grave doubts on this subject. I feel 
how rash it is to differ from these great authorities, 
to whom, with others, we owe all our knowledge. 
Those who think the natural geological record in 
any degree perfect, and who do not attach much 
weight to the facts and arguments of other kinds 
given in this volume, will undoubtedly at once re- 
ject my theory. For my part, following out Lyell's 
metaphor, I look at the natural geological record, as a 
history of the world imperfectly kept, and written in 
a changing dialect ; of this history we possess the last 
volume alone, relating only to two or three countries. 
Of this volume, only here and there a short chapter lias 


been preserved ; and of each page, only here and there 
a few lines. Each word of the slowly-changing lan- 
guage, in which the history is supposed to be written, 
being more or less different in the interrupted suc- 
cession of chapters, may represent the apparently 
abruptly changed forms of life, entombed in our con- 
secutive, but widely separated, formations. On this 
view, the difficulties above discussed are greatly dimi- 
nished, or even disappear. 



On the Geological Succession of Organic Beings. 

On the slow and successive appearance of new species — On their 
different rates of change — Species once lost do not reappear — 
Groups of species follow the same general rules in their appear- 
ance and disappearance as do single species — On Extinction — 
On simultaneous changes in the forms of life throughout the 
world — On the affinities of extinct species to each other and to 
living species — On the state of development of ancient forms — 
On the succession of the same types within the same areas — 
Summary of preceding and present chapters. 

Let us now see whether the several facts and rules 
relating to the geological succession of organic beings, 
better accord with the common view of the immutability 
of species, or with that of their slow and gradual modi- 
fication, through descent and natural selection. 

New species have appeared very slowly, one after 
another, both on the land and in the waters. Lyell 
has shown that it is hardly possible to resist the evidence 
on this head in the case of the several tertiary stages ; 
and every year tends to fill up the blanks between them, 
and to make the percentage system of lost and new 
forms more gradual. In some of the most recent beds, 
though undoubtedly of high antiquity if measured by 
years, only one or two species are lost forms, and only 
one or two are new forms, having here appeared for 
the first time, either locally, or, as far as we know, on 
the face of the earth. If we may trust the observations 
of Philippi in Sicily, the successive changes in the marine 
inhabitants of that island have been many and most 
gradual. The secondary formations are more broken ; 
but, as Bronn has remarked, neither the appearance 


nor disappearance of their many now extinct species lias 
been simultaneous in each separate formation. 

Species of different genera and classes have not 
changed at the same rate, or in the same degree. In 
the oldest tertiary beds a few living shells may still be 
found in the midst of a multitude of extinct forms. 
Falconer has given a striking instance of a similar fact, 
in an existing crocodile associated with many strange 
and lost mammals and reptiles in the sub-Himalayan 
deposits. The Silurian Lingula differs but little from 
the living species of this genus ; whereas most of the 
other Silurian Molluscs and all the Crustaceans have 
changed greatly. The productions of the land seem to 
change at a quicker rate than those of the sea, of which 
a striking instance has lately been observed in Switzer- 
land. There is some reason to believe that organisms, 
considered high in the scale of nature, change more 
quickly than those that are low : though there are ex- 
ceptions to this rule. The amount of organic change, 
as Pictet has remarked, does not strictly correspond 
with the succession of our geological formations ; so that 
between each two consecutive formations, the forms of 
life have seldom changed in exactly the same degree. 
Yet if we compare any but the most closely related for- 
mations, all the species will be found to have undergone 
some change. When a species has once disappeared 
from the face of the earth, we have reason to believe 
that the same identical form never reappears. The 
strongest apparent exception to this latter rule, is that 
of the so-called "colonies" of M. Barrande, which 
intrude for a period in the midst of an older formation, 
and 1 Ik mi allow the pre-existing fauna to reappear; but 
LyelTs explanation, namely, that it is a case of tempo- 
rary migration from a distinct geographical province, 
seems to me satisfactory. 



These several facts accord well with my theory. I 
believe in no fixed law of development, causing all the 
inhabitants of a country to change abruptly, or simul- 
taneously, or to an equal degree. The process of modi- 
fication must be extremely slow. The variability of each 
species is quite independent of that of all others. 
Whether such variability be taken advantage of by 
natural selection, and whether the variations be ac- 
cumulated to a greater or lesser amount, thus causing 
a greater or lesser amount of modification in the vary- 
ing species, depends on many complex contingencies, 
— on the variability being of a beneficial nature, on 
the power of intercrossing, on the rate of breeding, 
on the slowly changing physical conditions of the 
country, and more especially on the nature of the other 
inhabitants with winch the varying species comes into 
competition. Hence it is by no means surprising that 
one species should retain the same identical form much 
longer than others ; or, if changing, that it should change 
less. We see the same fact in geographical distribution ; 
for instance, in the land-shells and coleopterous insects 
of Madeira having come to differ considerably from their 
nearest allies on the continent of Em-ope, whereas the 
marine shells and birds have remained unaltered We 
can perhaps understand the apparently quicker rate of 
change in terrestrial and in more highly organised pro- 
ductions compared with marine and lower productions, 
by the more complex relations of the higher beingB 
to their organic and inorganic conditions of life, as 
explained in a former chapter. When many of the 
inhabitants of a country have become modified and im- 
proved, we can understand, on the principle of com- 
petition, and on that of the many all-important rela- 
tions of organism to organism, that any form which 
does not become in some degree modified and improved, 


will be liable to be exterminated. Hence we can 
see why all the species in the same region do at last, 
if we look to wide enough intervals of time, become 
modified ; for those which do not change will become 

In members of the same class the average amount of 
change, during long and equal periods of time, may, 
perhaps, be nearly the same ; but as the accumulation 
of long-enduring fossiliferous formations depends on 
great masses of sediment having been deposited on 
areas whilst subsiding, our formations have been almost 
necessarily accumulated at wide and irregularly inter- 
mittent intervals ; consequently the amount of organic 
change exhibited by the fossils embedded in con- 
secutive formations is not equal. Each formation, on 
this view, does not mark a new and complete act of 
creation, but only an occasional scene, taken almost at 
hazard, in a slowly changing drama. 

We can clearly understand why a species when once 
lost should never reappear, even if the very same con- 
ditions of life, organic and inorganic, should recur. 
For though the offspring of one species might be 
adapted (and no doubt this has occurred in innume- 
rable instances) to fill the exact place of another 
species in the economy of nature, and thus supplant it ; 
yet the two forms — the old and the new — would not 
be identically the same ; for both would almost cer- 
tainly inherit different characters from their distinct 
progenitors. For instance, it is just possible, if our 
fantail-pigeons were all destroyed, that fanciers, by 
striving during long ages for the same object, might 
make a new breed hardly distinguishable from our 
present fantail ; but if the parent rock-pigeon were also 
destroyed, and in nature we have every reason to believe 
that the parent-form will generally be supplanted and 



exterminated by its improved offspring, it is quite in- 
credible that a fantail, identical with the existing breed, 
could be raised from any other species of pigeon, or 
even from the other well-established races of the do- 
mestic pigeon, for the newly-formed fantail would be 
almost sure to inherit from its new progenitor some 
slight characteristic differences. 

Groups of species, that is, genera and families, follow 
the same general rules in their appearance and disap- 
r pearance as do single species, changing more or less 
I quickly, and in a greater or lesser degree. A group 
I does not reappear after it has once disappeared; or 
its existence, as long as it lasts, is continuous. I am 
aware that there are some apparent exceptions to this 
rule, but the exceptions are surprisingly few, so 
few, that E. Forbes, Pictet, and Woodward (though all 
strongly opposed to such views as I maintain) admit 
its truth ; and the rule strictly accords with my theory. 
For as all the species of the same group have descended 
from some one species, it is clear that as long as any 
sjjecies of the group have appeared in the long succession 
of ages, so long must its members have continuously 
existed, in order to have generated either new and 
modified or the same old and unmodified forms. Species 
of the genus Lingula, for instance, must have continu- 
ously existed by an unbroken succession of generations, 
from the lowest Silurian stratum to the present day. 

We have seen in the last chapter that the species 
of a group sometimes falsely appear to have come in 
abruptly; and I have attempted to give an explana- 
tion of this fact, which if true would have been fatal 
to my views. But such cases are certainly excep- 
tional; the general rule being a gradual increase in 
number, till the group reaches its maximum, and 
then, sooner or later, it gradually decreases. If the 

Chap. X. EXTINCTION. 317 

number of the species of a genus, or the number of 
the genera of a family, be represented by a vertical 
line of varying thickness, crossing the successive geo- 
logical formations in which the species are found, the 
line will sometimes falsely appear to begin at its lower 
end, not in a sharp point, but abruptly ; it then gradu- 
ally thickens upwards, sometimes keeping for a space 
of equal thickness, and ultimately thins out in the 
upper beds, marking the decrease and final extinction 
of the species. This gradual increase in number of the 
species of a group is strictly conformable with my 
theory ; as the species of the same genus, and the 
genera of the same family, can increase only slowly and 
progressively ; for the process of modification and the 
production of a number of allied forms must be slow 
and gradual, — one species giving rise first to two or 
three varieties, these being slowly converted into species, 
which in their turn produce by equally slow steps other 
species, and so on, like the branching of a great tree 
from a single stem, till the group becomes large. 

On Extinction. — We have as yet spoken only inci- 
dentally of the disappearance of species and of groups 
of species. On the theory of natural selection the ex- 
tinction of old forms and the production of new and im- 
proved forms are intimately connected together. The 
old notion of all the inhabitants of the earth having 
been swept away at successive periods by catastrophes, 
is very generally given up, even by those geologists, 
U_^ as Elie de Beaumont, Murchison, Barrande, &c, whose 
general views would naturally lead them to this conclu- 
sion. On the contrary, we have every reason to believe, 
from the study of the tertiary formations, that species 
and groups of species gradually disappear, one after 
another, first from one spot, then from another, and 


finally from the world. Both single species and whole 
groups of species last for very unequal periods ; some 
groups, as we have seen, having endured from the ear- 
liest known dawn of life to the present day ; some having 
disappeared before the close of the palaeozoic period. 
No fixed law seems to determine the length of time 
during which any single species or any single genus 
endures. There is reason to believe that the complete 
extinction of the species of a group is generally a 
slower process than their production : if the appearance 
and disappearance of a group of species be represented, 
as before, by a vertical line of varying thickness, the 
line is found to taper more gradually at its upper end, 
which marks the progress of extermination, than at its 
lower end, which marks the first appearance and in- 
crease in numbers of the species. In some cases, how- 
ever, the extermination of whole groups of beings, as of 
ammonites towards the close of the secondary period, 
has been wonderfully sudden. 

The whole subject of the extinction of species has 
been involved in the most gratuitous mystery. Some 
authors have even supposed that as the individual has a 
definite length of life, so have species a definite dura- 
tion. No one I think can have marvelled more at the 
extinction of species, than I have done. When I found 
in La Plata the tooth of a horse embedded with the 
remains of Mastodon, Megatherium, Toxodon, and other 
extinct monsters, which all co-existed with still living 
shells at a very late geological period, I was filled with 
astonishment ; for seeing that the horse, since its intro- 
duction by the Spaniards into South America, has run 
wild over the whole country and has increased in 
numbers at an unparalleled rate, I asked myself what 
could so recently have exterminated the former horse 
under conditions of life apparently so favourable. But 

Chap. X. EXTINCTION. 319 

how utterly groundless was my astonishment ! Pro- 
fessor Owen soon perceived that the tooth, though so 
like that of the existing horse, belonged to an extinct 
species. Had this horse been still living, but in some 
degree rare, no naturalist would have felt the least sur- 
prise at its rarity ; for rarity is the attribute of a vast 
number of species of all classes, in all countries. If 
we ask ourselves why this or that species is rare, we 
answer that something is unfavourable in its conditions 
of life ; but what that something is, we can hardly ever 
tell. On the supposition of the fossil horse still exist- 
ing as a rare species, we might have felt certain from 
the analogy of all other mammals, even of the slow- 
breeding elephant, and from the history of the natural- 
isation of the domestic horse in South America, that 
under more favourable conditions it would in a very few 
years have stocked the whole continent. But we could 
not have told what the unfavourable conditions were 
which checked its increase, whether some one or 
several contingencies, and at what period of the horse's 
life, and in what degree, they severally acted. If 
the conditions had gone on, however slowly, becom- 
ing less and less favourable, we assuredly should not 
have perceived the fact, yet the fossil horse would cer- 
tainly have become rarer and rarer, and finally extinct ; 
— its place being seized on by some more successful 

It is most difficult always to remember that the 
increase of every living being is constantly being 
checked by unperceived injurious agencies; and that 
these same unperceived agencies are amply sufficient to 
cause rarity, and finally extinction. We see in many 
cases in the more recent tertiary formations, that rarity 
precedes extinction; and we know that this has been 
the progress of events with those animals which have 


been exterminated, either locally or wholly, through 
man's agency. I may repeat what I published in 1845, 
namely, that to admit that species generally become 
1 rare before they become extinct — to feel no surprise at 
1 the rarity of a species, and yet to marvel greatly when 
it ceases to exist, is much the same as to admit that 
sickness in the individual is the forerunner of death — 
to feel no surprise at sickness, but when the sick man 
dies, to wonder and to suspect that he died by some 
unknown deed of violence. 

The theory of natural selection is grounded on the 
belief that each new variety, and ultimately each new 
species, is produced and maintained by having some 
advantage over those Avith which it comes into compe- 
tition ; and the consequent extinction of less-fa voured 
forms almost inevitably follows. It is the same with 
our domestic productions : when a new and slightly 
improved variety has been raised, it at first supplants 
the less improved varieties in the same neighbourhood ; 
when much improved it is transported far and near, 
like our short-horn cattle, and takes the place of other 
breeds in other countries. Thus the appearance of new 
forms and the disappearance of old forms, both natural 
and artificial, are bound together. In certain flourishing 
groups, the number of new specific forms which have 
been produced within a given time is probably greater 
than that of the old forms which have been extermi- 
nated ; but we know that the number of species has not 
gone on indefinitely increasing, at least during the later 
geological periods, so that looking to later times we 
may believe that the production of new forms lias caused 
the extinction of about the same number of old forms. 

The competition will generally be most severe, as 
formerly explained and illustrated by examples, between 
the forms which are most like each other in all respects, 

Chap. X. EXTINCTION. 321 

Hence the improved and modified descendants of a 
species will generally cause the extermination of the 
parent-species ; and if many new forms have been de- 
veloped from any one species, the nearest allies of that 
sjDecies, i. e. the species of the same genus, will be the 
most liable to extermination. Thus, as I believe, a 
number of new species descended from one species, that 
is a new genus, comes to supplant an old genus, belong- 
ing to the same family. But it must often have happened 
that a new species belonging to some one group will have 
seized on the place occupied by a species belonging to 
a distinct group, and thus caused its extermination ; 
and if many allied forms be developed from the success- 
ful intruder, many will have to yield their places ; and 
it will generally be allied forms, which will suffer 
from some inherited inferiority in common. But whe- 
ther it be species belonging to the same or to a 
distinct class, which yield their places to other species 
which have been modified and improved, a few of the 
sufferers may often long be preserved, from being 
fitted to some peculiar line of life, or from inhabiting 
some distant and isolated station, where they have 
escaped severe competition. For instance, a single 
species of Trigonia, a great genus of shells in the 
secondary formations, survives in the Australian seas ; 
and a few members of the great and almost extinct 
group of Ganoid fishes still inhabit our fresh waters. 
Therefore the utter extinction of a group is gene- 
rally, as we have seen, a slower process than its pro- 

With respect to the apparently sudden extermination 
of whole families or orders, as of Trilobites at the close 
of the palaeozoic period and of Ammonites at the close 
of the secondary period, we must remember what has 
been already said on the probable wide intervals of time 

p 3 


between our consecutive formations ; and in these inter- 
vals there may have been much slow extermination. 
Moreover, when by sudden immigration or by unusually 
rapid development, many species of a new group have 
taken possession of a new area, they will have exter- 
minated in a correspondingly rapid manner many of the 
old inhabitants ; and the forms winch thus yield their 
places will commonly be allied, for they will partake of 
some inferiority in common. 

Thus, as it seems to me, the manner in which single 
species and whole groups of species become extinct, 
accords well with the theory of natural selection. We 
need not marvel at extinction ; if we must marvel, let 
it be at our presumption in imagining for a moment 
that we understand the many complex contingencies, 
on wliich the existence of each species depends. If we 
forget for an instant, that each species tends to increase 
inordinately, and that some check is always in action, 
yet seldom perceived by us, the whole economy of 
nature will be utterly obscured. Whenever we can 
precisely say why tins species is more abundant in in- 
dividuals than that ; why this species and not another 
can be naturalised in a given country ; then, and not 
till then, we may justly feel surprise why we cannot 
account for the extinction of this particular species or 
group of species. 

On the Forms of Life changing almost simultaneously 
throughout the World. — Scarcely any paloeontological 
discovery is more striking than the fact, that the forms 
of life change almost simultaneously throughout the 
world. Thus our European Chalk formation can be 
recognised in many distant parts of the world, under 
the most different climates, where not a fragment oi* the 
mineral chalk itself can be found; namely, in North 


America, in equatorial South America, in Tierra del 
Fuego, at the Cape of Good Hope, and in the peninsula 
of India. For at these distant points, the organic re- 
mains in certain beds present an unmistakeable degree 
of resemblance to those of the Chalk. It is not that 
the same species are met with ; for in some cases not 
one species is identically the same, but they belong to 
the same families, genera, and sections of genera, and 
sometimes are similarly characterised in such trifling 
points as mere superficial sculpture. Moreover other 
forms, which are not found in the Chalk of Europe, but 
which occur in the formations either above or below, are 
similarly absent at these distant points of the world. In 
the several successive palaeozoic formations of Kussia, 
Western Europe and North America, a similar parallel- 
ism in the forms of life has been observed by several 
authors : so it is, according to Lyell, with the several 
European and North American tertiary deposits. Even 
if the few fossil species which are common to the Old 
and New Worlds be kept wholly out of view, the general 
parallelism in the successive forms of life, in the stages 
of the widely separated palaeozoic and tertiary periods, 
would still be manifest, and the several formations 
could be easily correlated. 

These observations, however, relate to the marine in- / 
habitants of distant parts of the world : we have not ' 
sufficient data to judge whether the productions of the I 
land and of fresh water change at distant points in the 
same parallel manner. We may doubt whether they 
have thus changed : if the Megatherium, Mylodon, 
Macrauchenia, and Toxodon had been brought to Europe 
from La Plata, without any information in regard to 
their geological position, no one would have suspected 
that they had coexisted with still living sea-shells ; but 
as these anomalous monsters coexisted with the Masto- 


don and Horse,' it might at least have been inferred that 
they had lived during one of the later tertiary stages. 

When the marine forms of life are spoken of as 
having changed simultaneously throughout the world, 
it must not be supposed that this expression relates to 
the same thousandth or hundred-thousandth year, or 
even that it has a very strict geological sense ; for if 
all the marine animals which live at the present clay in 
. Europe, and all those that lived in Europe during the 
pleistocene period (an enormously remote period as 
measured by years, including the whole glacial epoch), 
were to be compared with those now living in South 
America or in Australia, the most skilful naturalist 
would hardly be able to say whether the existing or the 
pleistocene inhabitants of Europe resembled most closely 
those of the southern hemisphere. So, again, several 
highly competent observers believe that the existing 
productions of the United States are more closely related 
to those which lived in Europe during certain later ter- 
tiary stages, than to those which now live here: and 
if this be so, it is evident that fossiliferous beds de- 
posited at the present day on the shores of North 
America would hereafter be liable to be classed with 
somewhat older European beds. Nevertheless, looking 
to a remotely future epoch, there can, I think, be little 
doubt that all the more modern marine format ions, 
namely, the upper pliocene, the pleistocene and strictly 
modern beds, of Europe, North and South America, and 
Australia, from containing fossil remains in some degree 
allied, and from not including those forms which are 
only found in the older underlying deposits, would be 
correctly ranked as simultaneous in a geological sense. 

The fact of the forms of life changing simultaneously, 
in the above large sense, at distant parts el' the world, 
lias greatly struck those admirable observers, Mil, 


de Verneuil and d'Archiac. After referring to the 
parallelism of the palaeozoic forms of life in various 
parts of Europe, they add, " If struck by this strange 
sequence, we turn our attention to North America, and 
there discover a series of analogous phenomena, it will 
appear certain that all these modifications of species, 
their extinction, and the introduction of new ones, can- 
not be owing to mere changes in marine currents or 
other causes more or less local and temporary, but de- 
pend on general laws which govern the whole animal 
kingdom." M. Barrande has made forcible remarks to 
precisely the same effect. It is, indeed, quite futile to 
look to changes of currents, climate, or other physical 
conditions, as the cause of these great mutations in the 
forms of life throughout the world, under the most dif- 
ferent climates. We must, as Barrande has remarked, 
look to some special law. We shall see this more clearly 
when we treat of the present distribution of organic 
beings, and find how slight is the relation between the 
physical conditions of various countries, and the nature 
of their inhabitants. 

This great fact of the parallel succession of the forms 
of life throughout the world, is explicable on the theory 
of natural selection. New species are formed by new 
varieties arising, which have some advantage over 
older forms ; and those forms, which are already domi- 
nant, or have some advantage over the other forms in 
their own country, would naturally oftenest give rise to 
new varieties or incipient species ; for these latter must 
be victorious in a still higher degree in order to be pre- 
served and to survive. We have distinct evidence on 
this head, in the plants which are dominant, that is, 
which arc commonest in their own homes, and are most 
widely diffused, having produced the greatest number 
of new varieties. It is also natural that the domi- 


nant, varying, and far-spreading species, which already 
have invaded to a certain extent the territories of other 
species, should be those which would have the best 
chance of spreading still further, and of giving rise in 
new countries to new varieties and species. The process 
of diffusion may often be very slow, being dependent 
on climatal and geographical changes, or on strange 
accidents, but in the long run the dominant forms will 
generally succeed in spreading. The diffusion would, it 
is probable, be slower with the terrestrial inhabitants of 
distinct continents than with the marine inhabitants of 
the continuous sea. We might therefore expect to find, 
as we apparently do find, a less strict degree of parallel 
succession in the productions of the land than of the sea. 
Dominant species spreading from any region might 
encounter still more dominant sj)ecies, and then their 
triumphant course, or even their existence, would cease. 
We know not at all precisely what are all the conditions 
most favourable for the multiplication of new and domi- 
nant species ; but we can, I think, clearly see that a 
number of individuals, from giving a better chance of 
the appearance of favourable variations, and that severe 
competition with many already existing forms, would be 
highly favourable, as would be the power of spreading 
into new territories. A certain amount of isolation, 
recurring at long intervals of time, would probably be 
also favourable, as before explained. One quarter of 
the world may have been most favourable for the pro- 
duction of new and dominant species on the land, and 
another for those in the waters of the sea. If two great 
regions had been for a long period favourably circum- 
stanced in an equal degree, whenever their Inhabitants 
met, the battle would be prolonged and severe ; and 
some from one birthplace and some from the other 
might be victorious. But in the course of time, the 


forms dominant in the highest degree, wherever pro- 
duced, would tend everywhere to prevail. As they pre- 
vailed, they would cause the extinction of other and 
inferior forms ; and as these inferior forms would be 
allied in groups by inheritance, whole groups would 
tend slowly to disappear ; though here and there a 
single member might long be enabled to survive. 

Thus, as it seems to me, the parallel, and, taken in a 
large sense, simultaneous, succession of the same forms 
of life throughout the world, accords well with the prin- 
ciple of new species having been formed by dominant 
species spreading widely and varying ; the new species 
thus produced being themselves dominant owing to in- 
heritance, and to having already had some advantage 
over their parents or over other species ; these again 
spreading, varying, and producing new species. The 
forms which are beaten and which yield their places to 
the new and victorious forms, will generally be allied in 
groups, from inheriting some inferiority in common; 
and therefore as new and improved groups spread 
throughout the world, old groups will disappear from 
the world ; and the succession of forms in both ways 
will everywhere tend to correspond. 

There is one other remark connected with this subject 
worth making. I have given my reasons for believ- 
ing that all our greater fossiliferous formations were 
deposited during periods of subsidence ; and that 
blank intervals of vast duration occurred during the 
periods when the bed of the sea was either station- 
ary or rising, and likewise when sediment was not 
thrown down quickly enough to embed and preserve 
organic remains. During these long and blank inter- 
vals I suppose that the inhabitants of each region 
underwent a considerable amount of modification and 
extinction, and that there was much migration from 


other parts of the world. As we have reason to 
believe that large areas are affected by the same move- 
ment, it is probable that strictly contemporaneous for- 
mations have often been accumulated over very wide 
spaces in the same quarter of the world ; but we are 
far from having any right to conclude that this has in- 
variably been the case, and that large areas have invari- 
ably been affected by the same movements. When two 
formations have been deposited in two regions during 
nearly, but not exactly the same period, we should find 
in both, from the causes explained in the foregoing 
paragraphs, the same general succession in the forms of 
life ; but the species would not exactly correspond ; for 
there will have been a little more time in the one region 
than in the other for modification, extinction, and im- 

I suspect that cases of this nature have occurred in 
Euixype. Mr. Prestwich, in his admirable Memoirs on 
the eocene deposits of England and France, is able to 
draw a close general parallelism between the successive 
stages in the two countries; but when he compares 
certain stages in England with those in France, although 
he finds in both a curious accordance in the numbers 
of the species belonging to the same genera, yel the 
species themselves differ in a manner very difficult to 
account for, considering the proximity of the two areas, 
— unless, indeed, it be assumed that an isthmus separated 
two seas inhabited by distinct, but contemporaneous, 
faunas. Ly ell has made similar observations on some of 
the later tertiary formations. Barrande, also, shows that 
there is a striking general parallelism in the successive 
Silurian deposits of Bohemia and Scandinavia ; never- 
theless he finds a surprising amount of difference in 
the species. If the several formal ions in these re- 
gions have not been deposited during the same exact 


periods, — a formation in one region often corresponding 
with a blank interval in the other, — and if in both 
regions the species have gone on slowly changing 
daring the accumulation of the several formations and 
during the long intervals of time between them ; in 
this- case, the several formations in the two regions 
could be arranged in the same order, in accordance 
with the general succession of the form of life, and the 
order would falsely appear to be strictly parallel ; never- 
theless the species would not all be the same in the 
apparently corresponding stages in the two regions. 

On the Affinities of extinct Species to each other, and to 
living forms. — Let us now look to the mutual affinities 
of extinct and living species. They all fall into one 
grand natural system ; and this fact is at once explained 
on the principle of descent. The more ancient any 
form is, the more, as a general rule, it differs from living 
forms. But, as Buckland long ago remarked, all fossils 
can be classed either in still existing groups, or between 
them. That the extinct forms of life help to fill up the 
wide intervals between existing genera, families, and 
orders, cannot be disputed. For if we confine our atten- 
tion either to the living or to the extinct alone, the 
series is far less perfect than if we combine both into 
one general system. With respect to the Vertebrata, 
whole pages could be filled with striking illustrations 
from our great palaeontologist, Owen, showing how ex- 
tinct animals fall in between existing groups. Cuvier 
ranked the Ruminants and Pachyderms, as the two most 
distinct orders of mammals ; but Owen has discovered 
so many fossil links, that he has had to alter the whole 
classification of these two orders ; and has placed certain 
pachyderms in the same sub-order with ruminants : for 
example, he dissolves by fine gradations the apparently 


wide difference between the pig and the camel. In 
regard to the Invertebrata, Barrande, and a higher 
authority could not be named, asserts that he is every- 
day taught that palaeozoic animals, though belonging to 
the same orders, families, or genera with those living 
at the present day, were not at this early epoch limited 
in such distinct groups as they now are. 

Some writers have objected to any extinct species or 
group of species being considered as intermediate be- 
tween living species or groups. If by this term it is 
meant that an extinct form is directly intermediate in 
all its characters between two living forms, the objec- 
tion is probably valid. But I apprehend that in a 
perfectly natural classification many fossil species would 
have to stand between living species, and some extinct 
genera between living genera, even between genera 
belonging to distinct families. The most common case, 
especially with respect to very distinct groups, such as 
fish and reptiles, seems to be, that supposing them to be 
distinguished at the present day from each other by a 
dozen characters, the ancient members of the same two 
groups would be distinguished by a somewhat lesser 
number of characters, so that the two groups, though 
formerly quite distinct, at that period made some smal l 
approach to each other. 

It is a common belief that the more ancient a form 
is, by so much the more it tends to connect by some of 
its characters groups now widely separated from each 
other. This remark no doubt must be restricted to 
those groups which have undergone much change in the 
course of geological ages ; and it would be difficult to 
prove the truth of the proposition, for every now and 
then even a living animal, as the Lepidosiron, is dis- 
covered having affinities directed towards very distinct 
groups. Yet if we compare the older Reptiles and 


Batrachians, the older Fish, the older Cephalopods, and 
the eocene Mammals, with the more recent members 
of the same classes, we must admit that there is some 
truth in the remark. 

Let us see how far these several facts and inferences 
accord with the theory of descent with modification. As 
the subject is somewhat complex, I must request the 
reader to turn to the diagram in the fourth chapter. 
We may suppose that the numbered letters represent 
genera, and the dotted lines diverging from them 
the species in each genus. The diagram is much too 
simple, too few genera and too few species being 
given, but this is unimportant for us. The horizontal 
lines may represent successive geological formations, 
and all the forms beneath the uppermost line may 
be considered as extinct. The three existing genera, 
a u , q u , p u , will form a small family ; b u and / 14 a 
closely allied family or sub-family ; and o u e u , m H , a 
third family. These three families, together with the 
many extinct genera on the several lines of descent 
diverging from the parent-form A, will form an order ; 
for all will have inherited something in common 
from their ancient and common progenitor. On the 
principle of the continued tendency to divergence of 
character, which was formerly illustrated by this dia- 
gram, the more recent any form is, the more it will 
generally differ from its ancient progenitor. Hence we 
con understand the rule that the most ancient fossils 
differ most from existing forms. We must not, how- 
ever, assume that divergence of character is a necessary 
contingency ; it depends solely on the descendants from 
a species being thus enabled to seize on many and dif- 
ferent places in the economy of nature. Therefore it is 
quite possible, as we have seen in the case of some 
Silurian forms, that a species might go on being slightly 


modified in relation to its slightly altered conditions of 
life, and yet retain throughout a vast period the same 
general characteristics. This is represented in the dia- 
gram by the letter f u . 

All the many forms, extinct and recent, descended 
from A, make, as before remarked, one order ; and this 
order, from the continued effects of extinction and 
divergence of character, has become divided into se- 
veral sub-families and families, some of which are 
supposed to have perished at different periods, and some 
to have endured to the present day. 

By looking at the diagram we can see that if many 
of the extinct forms, supposed to be embedded in the 
successive formations, were discovered at several points 
low down in the series, the three existing families on the 
uppermost line would be rendered less distinct from each 
other. If, for instance, the genera a 1 , a 5 , a 1 °,/ 8 , m 8 , m 6 , m 9 , 
were disinterred, these three families would be so closely 
linked together that they probably would have to be 
united into one great family, in nearly the same manner 
as has occurred with ruminants and pachyderms. Yet 
he who objected to call the extinct genera, which thus 
linked the living genera of three families together, inter- 
mediate in character, would be justified, as they are 
intermediate, not directly, but only by a long and cir- 
cuitous course through many widely different forms. If 
many extinct forms were to be discovered above one 
of the middle horizontal lines or geological formations 
« — for instance, above No. VI. — but none from beneath 
this line, then only the two families on the left hand 
(namely, a u , &c, and b u , &c.) would have to be united 
into one family; and the two other families (namely, 
a u to/ 14 now including five genera, and o 14 to m u ) Mould 
yet remain distinct. These two families, however, would 
be less distinct from each other than they were before the 


discovery of the fossils. If, for instance, we suppose the 
existing genera of the two families to differ from each 
other by a dozen characters, in this case the genera, at 
the early period marked VI., would differ by a lesser 
number of characters ; for at this early stage of descent 
"they have not diverged in character from the common 
progenitor of the order, nearly so much as they sub- 
sequently diverged. Thus it comes that ancient and 
extinct genera are often in some slight degree inter- 
mediate in character between their modified descendants, 
or between their collateral relations. 

In nature the case will be far more complicated than 
is represented in the diagram ; for the groups will have 
been more numerous, they will have endured for ex- 
tremely unequal lengths of time, and will have been 
modified in various degrees. As we possess only the last 
volume of the geological record, and that in a very broken 
condition, we have no right to expect, except in very 
rare cases, to fill up wide intervals in the natural system, 
and thus unite distinct families or orders. All that 
we have a right to expect, is that those groups, which 
have within known geological periods undergone much 
modification, should in the older formations make some 
slight approach to each other ; so that the older mem- 
bers shoidd differ less from each other in some of their 
characters than do the existing members of the same 
groups ; and this by the concurrent evidence of our best 
paleontologists seems frequently to be the case. 

Thus, on the theory of descent with modification, the 
main facts with respect to the mutual affinities of the 
extinct forms of life to each other and to living forms, 
seem to me explained in a satisfactory manner. And 
they are wholly inexplicable on any other view. 

On this same theory, it is evident that the fauna of 
any great period in the earth's lnstory will be inter- 


mediate in general character between that which pre- 
ceded and that which succeeded it. Thus, the species 
which lived at the sixth great stage of descent in the 
diagram are the modified offspring of those which lived 
at the fifth stage, and are the parents of those which 
became still more modified at the seventh stage ; hence 
they could hardly fail to be nearly intermediate in 
character between the forms of life above and below. 
We must, however, allow for the entire extinction of 
some preceding forms, and for the coming in of quite 
new forms by immigration, and for a large amount of 
modification, during the long and blank intervals be- 
tween the successive formations. Subject to these allow- 
ances, the fauna of each geological period undoubtedly 
is intermediate in character, between the preceding and 
succeeding faunas. I need give only one instance, 
namely, the manner in which the fossils of the Devonian 
system, when this system was first discovered, were at 
once recognised by palaeontologists as intermediate in 
character between those of the overlying carboni- 
ferous, and underlying Silurian system. But each 
fauna is not necessarily exactly intermediate, as 
unequal intervals of time have elapsed between con- 
secutive formations. 

It is no real objection to the truth of the statement, 
that the fauna of each period as a whole is nearly in- 
termediate in character between the preceding and 
succeeding faunas, tliat certain genera offer exceptions 
to the rule. For instance, mastodons and elephants, 
when arranged by Dr. Falconer in two series, first 
according to their mutual affinities and then according 
to their periods of existence, do not accord in arrange* 
ment. The species extreme in character are not the 
oldest, or the most recent; nor are those which are 
intermediate in character, intermediate in age. But 


supposing for an instant, in this and other such cases, 
that the record of the first appearance and disappearance 
of the species was perfect, we have no reason to believe 
that forms successively produced necessarily endure for 
corresponding lengths of time : a very ancient form 
might occasionally last much longer than a form else- 
where subsequently produced, especially in the case of 
terrestrial productions inhabiting separated districts. 
To compare small things with great: if the principal 
living and extinct races of the domestic pigeon were 
arranged as well as they could be in serial affinity, this 
arrangement would not closely accord with the order in 
time of their production, and still less with the order of 
their disappearance ; for the parent rock-pigeon now 
lives ; and many varieties between the rock-pigeon and 
the carrier have become extinct; and carriers which 
are extreme in the important character of length of 
beak originated earlier than short-beaked tumblers, 
which are at the opposite end of the series in this same 

Closely connected with the statement, that the or- 
ganic remains from an intermediate formation are in 
some degree intermediate in character, is the fact, 
insisted on by all palaeontologists, that fossils from two 
consecutive formations are far more closely related to 
each other, than are the fossils from two remote forma- 
tions. Pictet gives as a well-known instance, the 
general resemblance of the organic remains from the 
several stages of the chalk formation, though the species 
are distinct in each stage. This fact alone, from its 
generality, seems to have shaken Professor Pictet in his 
firm belief in the immutability of species. He who is 
acquainted with the distribution of existing species over 
the globe, will not attempt to account for the close 
resemblance of the distinct species in closely consecutive 


formations, by the physical conditions of the ancient 
areas having remained nearly the same. Let it be 
remembered that the forms of life, at least those in- 
habiting the sea, have changed almost simultaneously 
throughout the world, and therefore under the most 
different climates and conditions. Consider the pro- 
digious vicissitudes of climate during the pleistocene 
period, which includes the whole glacial period, and 
note how little the specific forms of the inhabitants of 
the sea have been affected. 

On the theory of descent, the full meaning of the 
fact of fossil remains from closely consecutive forma- 
tions, though ranked as distinct species, being closely 
related, is obvious. As the accumulation of each for- 
mation has often been interrupted, and as long blank 
intervals have intervened between successive formations. 
we ought not to expect to find, as I attempted to show 
in the last chapter, in any one or two formations all the 
intermediate varieties between the species which ap- 
peared at the commencement and close of these periods ; 
but we ought to find after intervals, very long as 
measured by years, but only moderately long as 
measured geologically, closely allied forms, or, as they 
have been called by some authors, representative spe- 
cies; and these we assuredly do find. We find, in 
short, such evidence of the slow and scarcely sensible 
mutation of specific forms, as we have a just right to 
expect to find. 

On the state of Development of Ancient Forms. — There 
has been much discussion whether recent forms are 
more highly developed than ancient. I will not here 
enter on this subject, for naturalists have not as yet 
defined to each other's satisfaction what is meant by 
high and low forms. But in one particular sense the 


more recent forms must, on my theory, be higher than 
the more ancient; for each new species is formed by 
having had some advantage in the struggle for life 
over other and preceding forms. If under a nearly 
similar climate, the eocene inhabitants of one quarter 
of the world were put into competition with the exist- 
ing inhabitants of the same or some other quarter, 
the eocene fauna or flora would certainly be beaten 
and exterminated; as would a secondary fauna by an 
eocene, and a palaeozoic fauna by a secondary fauna. 
I do not doubt that this process of improvement has 
affected in a marked and sensible manner the organ- 
isation of the more recent and victorious forms of life, 
in comparison with the ancient and beaten forms ; but 
I can see no way of testing this sort of progress. 
Crustaceans, for instance, not the highest in their own 
class, may have beaten the highest molluscs. From 
the extraordinary manner in which European pro- 
ductions have recently spread over New Zealand, and 
have seized on places which must have been previously 
occupied, we may believe, if all the animals and plants 
of Great Britain were set free in New Zealand, that in 
the course of time a multitude of British forms would 
become thoroughly naturalized there, and would exter- 
minate many of the natives. On the other hand, 
from what we see now occurring in New Zealand, and 
from hardly a single inhabitant of the southern hemi- 
sphere having become wild in any part of Europe, 
we may doubt, if all the productions of New Zealand 
were set free in Great Britain, whether any consider- 
able number would be enabled to seize on places now 
occupied by our native plants and animals. Under tins 
point of view, the productions of Great Britain may be 
said to be higher than those of New Zealand. Yet 
the most skilful naturalist from an examination of the 



species of the two countries could- not have foreseen 
this result. 

Agassiz insists that ancient animals resemble to a 
certain extent the embryos of recent animals of the 
same classes ; or that the geological succession of 
extinct forms is in some degree parallel to the embryo- 
logical development of recent forms. I must follow 
Pictet and Huxley in thinking that the truth of this 
doctrine is very far from proved. Yet I fully expect to 
see it hereafter confirmed, at least in regard to subordi- 
nate groups, which have branched off from each other 
within comparatively recent times. For this doctrine 
of Agassiz accords well with the theory of natural 
selection. In a future chapter I shall attempt to show 
that the adult differs from its embryo, owing to varia- 
tions supervening at a not early age, and being inhe- 
rited at a corresponding age. This process, whilst it 
leaves the embryo almost unaltered, continual ly adds, in 
the course of successive generations, mure and more 
difference to the adult. 

Thus the embryo comes to be left a's a sort of picture, 
preserved by nature, of the ancient and less modified 
condition of each animal This view may be true, and 
yet it may never be capable of full proof. Seeing, for 
instance, that the oldest known mammals, reptiles, and 
fish strictly belong to their own proper classes, though 
some of these old forms are in a .slight degree Less dis- 
tinct from each other than are the typical members el' 
the same groups at the present day, it would be vain to 
look for animals having the common embryologicaJ 

character of the Yertebrata. until beds far beneath the 
lowest Silurian strata are discovered — a discovery el' 
which the chance is very small. 

On the Succession of the same Type* within the same 


areas, during the later tertiary periods. — Mr. Clift many 
years ago showed that the fossil mammals from the 
Australian caves were closely allied to the living mar- 
supials of that continent. In South America, a similar 
relationship is manifest, even to an uneducated eye, 
in the gigantic pieces of armour like those of the arma- 
dillo, found in several parts of La Plata ; and Professor 
Owen has shown in the most striking manner that most 
of the fossil mammals, buried there in such numbers, 
are related to South American types. This relation- 
ship is even more clearly seen in the wonderful collec- 
tion of fossil bones made by MM. Lund and Clausen in 
the caves of Brazil. I was so much impressed with 
these facts that I strongly insisted, in 1839 and 1845, 
on this "law of the succession of types," — on "this won- 
derful relationship in the same continent between the 
dead and the living." Professor Owen has subsequently 
extended the same generalisation to the mammals of 
the Old World. We see the same law in this author's 
restorations of the extinct and gigantic birds of New 
Zealand. We see it also in the birds of the caves of 
Brazil. Mr. Woodward has shown that the same law 
holds good with sea-shells, but from the wide distribu- 
tion of most genera of molluscs, it is not well displayed 
by them. Other cases could be added, as the relation 
between the extinct and living land-shells of Madeira ; 
and between the extinct and living brackish-water shells 
of the Aralo-Caspian Sea. 

Now what does this remarkable law of the succes- 
sion of the same types within the same areas mean ? 
He would be a bold man, who after comparing the pre- 
sent climate of Australia and of parts of South America 
under the same latitude, would attempt to account, on 
the one hand, by dissimilar physical conditions for the 
dissimilarity of the inhabitants of these two continents. 

Q 2 


and, on the other hand, by similarity of conditions, for 
the uniformity of the same types in each during the 
later tertiary periods. Nor can it be pretended that it 
is an immutable law that marsupials should have been 
chiefly or solely produced in Australia ; or that Eden- 
tata and other American types should have been solely 
produced in South America. For we know that Europe 
in ancient times was peopled by numerous marsupials ; 
and I have shown in the publications above alluded to, 
that in America the law of distribution of terrestrial 
mammals was formerly different from what it now is. 
North America formerly partook strongly of the pre- 
sent character of the southern half of the continent ; 
and the southern half was formerly more closely allied, 
than it is at present, to the northern half. In a similar 
manner we know from Falconer and Cautley's dis- 
coveries, that northern India was formerly more closely 
related in its mammals to Africa than it is at the pre- 
sent time. Analogous facts could be given in relation 
to the distribution of marine animals. 

On the theory of descent with modification, the great 
law of the long enduring, but not immutable, succession 
of the same types within the same areas, is at once 
explained ; for the inhabitants of each quarter of the 
world will obviously tend to leave in that quarter, 
during the next succeeding period of time, closely 
allied though in some degree modified descendants. If 
the inhabitants of one continent formerly differed 
greatly from those of another continent, so will their 
modified descendants still differ in nearly the same 
manner and degree. But after very long intervals of 
time and after great geographical changes, permitting 
much inter-hiigration, the feebler will yield to the 
more dominant forms, and there will be nothing im- 
mutable in the laws of past and present distribution. 


It may be asked in ridicule, whether I suppose that 
the megatherium and other allied huge monsters have 
left behind them in South America the sloth, armadillo, 
and anteater, as their degenerate descendants. This 
cannot for an instant be admitted. These huge ani- 
mals have become wholly extinct, and have left no pro- 
geny. But in the caves of Brazil, there are many 
extinct species which are closely allied in size and in 
other characters to the species still living in South 
America ; and some of these fossils may be the actual 
progenitors of living species. It must not be for- 
gotten that, on my theory, all the species of the same 
genus have descended from some one species ; so that 
if six genera, each having eight species, be found in one 
geological formation, and in the next succeeding forma- 
tion there be six other allied or representative genera 
with the same number of species, then we may con- 
clude that only one species of each of the six older 
genera has left modified descendants, constituting the 
six new genera. The other seven species of the old 
genera have all died out and have left no progeny. Or, 
which would probably be a far commoner case, two or 
three species of two or three alone of the six older 
genera will have been the parents of the six new 
genera ; the other old species and the other whole 
genera having become utterly extinct. In failing 
orders, with the genera and species decreasing in 
numbers, as apparently is the case of the Edentata of 
South America, still fewer genera and species will have 
left modified blood-descendants. 

Summary of the preceding and present Chapters. — I 
have attempted to show that the geological record is 
extremely imperfect ; that only a small portion of the 
globe has been geologically explored with care ; that 


only certain classes of organic beings have been largely 
preserved in a fossil state ; that the number both of 
specimens and of species, preserved in onr museums, is 
absolutely as nothing compared with the incalculable 
number of generations which must have passed away 
even during a single formation; that, owing to sub- 
sidence being necessary for the accumulation of fossili- 
ferous deposits thick enough to resist future degradation, 
enormous intervals of time have elapsed between the 

successive formations; that there has probably I n 

more extinction during the periods of subsidence, and 
more variation during the periods of elevation, and 
during the latter the record will have ben least per- 
fectly kept ; that each single formation has not been 
continuously deposited ; that the duration of each 
formation is, perhaps, short compared with the average 
duration of specific forms; that migration lias played 
an important part in the first appearance of new forma 
in any one area and formation; that widely ranging 
species are those which have varied most, and have 
oftenest given rise to new species; and that varieties 
have at first often been local. All these causes taken 
conjointly, must have tended to make the geological 
record extremely imperfect, and will to a large extent 
explain why we do not find interminable varieties, con- 
necting together all the extinct and existing forms of 
life by the finest graduated steps. 

He who rejects these views on the nature of the 
geological record, will rightly reject my whole theory. 
For he may ask in vain where are the numberless 
transitional links which must formerly have connected 
the closely allied or representative species, found in 
the several stages of the same great formation, lie 
may disbelieve in the enormous intervals of time which 
have elapsed between our consecutive formations; he 

Chap. X. SUMMAKY. 343 

may overlook liow important a part migration must 
have played, when the formations of any one great 
region alone, as that of Europe, are considered ; he may 
urge the apparent, but often falsely apparent, sudden 
coming in of whole groups of species. He may ask 
where are the remains of those infinitely numerous 
organisms which must have existed long before the 
first bed of the Silurian system was deposited : I can 
answer this latter question only hypo the tically, by say- 
ing that as far as we can see, where our oceans now 
extend they have for an enormous period extended, and 
where our oscillating continents now stand they have 
stood ever since the Silurian epoch; but that long 
before that period, the world may have presented a 
wholly different aspect ; and that the older continents, 
formed of formations older than any known to us, may 
now all be in a metamorphosed condition, or may lie 
buried under the ocean. 

Passing from these difficulties, all the other great 
leading facts in palaeontology seem to me simply to 
follow on the theory of descent with modification 
through natural selection. We can thus understand 
how it is that new species come in slowly and succes- 
sively ; how species of different classes do not neces- 
sarily change together, or at the same rate, or in the 
same degree ; yet in the long run that all undergo 
modification to some extent. The extinction of old 
forms is the almost inevitable consequence of the pro- 
duction of new forms. We can understand why when 
a species has once disappeared it never reappears. 
Groups of species increase in numbers slowly, and 
endure for unequal periods of time ; for the process of 
modification is necessarily slow, and depends on many 
complex contingencies. The dominant species of the 
larger dominant groups tend to leave many modified 


descendants, and thus new sub-groups and groups are 
formed. As these are formed, the species of the less 
vigorous groups, from their inferiority inherited from a 
common progenitor, tend to become extinct together, and 
to leave no modified offspring on the face of the earth. 
But the utter extinction of a whole group of species 
may often be a very slow process, from the survival of a 
few descendants, lingering in protected and isolated 
f situations. When a group has once wholly disappeared, 
| it does not reappear ; for the link of generation has 
been broken. 

We can understand how the spreading of the domi- 
nant forms of life, which are those that oftenest vary, 
will in the long run tend to people the world with 
allied, but modified, descendants ; and these will gene- 
rally succeed in taking the places of those groups of 
species which are their inferiors in the struggle for 
existence. Hence, after long intervals of time, the 
productions of the world will appear to have changed 

We can understand how it is that all the forms of 
life, ancient and recent, make together one grand 
system ; for all are connected by generation. We can 
understand, from the continued tendency to divergence 
of character, why the more ancient a form is, the more 
it generally differs from those now living. Why ancient 
and extinct forms often tend to fill up gaps between 
existing forms, sometimes blending two groups previ- 
ously classed as distinct into one ; but more commonly 
only bringing them a little closer together. The more 
ancient a form is, the more often, apparently, it dis- 
plays characters in some degree intermediate between 
groups now distinct ; for the more ancient a form is, 
the more nearly it will be related to, and consequently 
resemble, the common progenitor of groups, since be- 

Chap. x. SUMMARY. 345 

come widely divergent. Extinct forms are seldom 
directly intermediate between existing forms ; but are 
intermediate only by a long and circuitous course 
through many extinct and very different forms. We 
can clearly see why the organic remains of closely con- 
secutive formations are more closely allied to each 
other, than are those of remote formations; for the 
forms are more closely linked together by generation : 
we can clearly see why the remains of an intermediate 
formation are intermediate in character. 

The inhabitants of each successive period in the 
world's history have beaten their predecessors in the 
race for life, and are, in so far, higher in the scale of 
nature ; and this may account for that vague yet ill- 
defined sentiment, felt by many palaeontologists, that 
organisation on the whole has progressed. If it should 
hereafter be proved that ancient animals resemble to 
a certain extent the embryos of more recent animals 
of the same class, the fact will be intelligible. The 
succession of the same types of structure within the 
same areas during the later geological periods ceases to 
be mysterious, and is simply explained by inheritance. 

If then the geological record be as imperfect as I 
believe it to be, and it may at least be asserted that 
the record cannot be proved to be much more perfect, 
the main objections to the theory of natural selection 
are greatly diminished or disappear. On the other 
hand, all the chief laws of palaeontology plainly pro- 
claim, as it seems to me, that species have been pro- 
duced by ordinary generation : old forms having been 
supplanted by new and improved forms of life, produced 
by the laws of variation still acting round us, and pre- 
served by Natural Selection. 




Geographical Distribution. 

Present distribution cannot be accounted for by differences in phy- 
sical conditions — Importance of barriers — Affinity of the pro- 
ductions of the same continent — Centres of creation — Means 
of dispersal, by changes of climate and of the level of the land, 
and by occasional means — Dispersal during the Glacial period 
co-extensive with the world. 

In considering the distribution of organic beings over 
the face of the globe, the first great fact which strikes 
us is, that neither the similarity nor the dissimilarity 
of the inhabitants of various regions can be accounted 
for by their climatal and other physical conditions. Of 
late, almost every author who has studied the subject 
has come to this conclusion. The case of America 
alone would almost suffice to prove its truth : for if we 
exclude the northern parts where the circumpolar land 
is almost continuous, all authors agree that one of the 
most fundamental divisions in geographical distribution 
is that between the New and Old Worlds ; yet if we 
travel over the vast American continent, from the 
central parts of the United States to its extreme 
southern point, we meet with the most diversified con- 
ditions ; the most humid districts, arid deserts, lofty 
mountains, grassy plains, forests, marshes, lakes, and 
great rivers, under almost every temperature. There is 
hardly a climate or condition in the Old World which 
cannot be paralleled in the New — at least as closely 
as the same species generally require ; for it is a most 
rare case to find a group of organisms confined to any 
small spot, having conditions peculiar in only a slight 


degree ; for instance, small areas in the Old World 
could be pointed out hotter than any in the New 
World, yet these are not inhabited by a peculiar fauna 
or flora. Notwithstanding this parallelism in the con- 
ditions of the Old and New Worlds, how widely differ- 
ent are their living productions ! 

In the southern hemisphere, if we compare large 
tracts of land in Australia, South Africa, and western 
South America, between latitudes 25° and 35°, we shall 
find parts extremely similar in all their conditions, yet 
it would not be possible to point out three faunas and 
floras more utterly dissimilar. Or again we may com- 
pare the productions of South America south of lat. 
35° with those north of 25°, which consequently inhabit 
a considerably different climate, and they will be found 
incomparably more closely related to each other, than 
they are to the productions of Australia or Africa under 
nearly the same climate. Analogous facts could be 
given with respect to the inhabitants of the sea. 

A second great fact which strikes us in our general l_ 
review is, that barriers of any kind, or obstacles to free 
migration, are related in a close and important manner 
to the differences between the productions of various 
regions. We see this in the great difference of nearly 
all the terrestrial productions of the New and Old 
Worlds, excepting in the northern parts, where the 
land almost joins, and where, under a slightly different 
climate, there might have been free migration for tin 4 
northern temperate forms, as there now is for the 
strictly arctic productions. We see the same fact in 
the great difference between the inhabitants of Aus- 
tralia, Africa, and South America under the same lati- 
tude: for these countries are almost as much isolated 
from each other as is possible. On each continent, 
also, we see the same fact ; for on the opposite sides of 


lofty and continuous mountain-ranges, and of great 
deserts, and sometimes even of large rivers, we find 
different productions ; though as mountain-chains, 
deserts, &c, are not as impassable, or likely to have 
endured so long as the oceans separating continents, 
the differences are very inferior in degree to those cha- 
racteristic of distinct continents. 

Turning to the sea, we find the same law. No two 
marine faunas are more distinct, with hardly a fish, 
shell, or crab in common, than those of the eastern and 
western shores of South and Central America; yet 
these great faunas are separated only by the narrow, 
but impassable, isthmus of Panama. Westward of the 
shores of America, a wide space of open ocean extends, 
with not an island as a halting-place for emigrants ; 
here we have a barrier of another kind, and as soon as 
this is passed we meet in the eastern islands of the 
Pacific, with another and totally distinct famia. So 
that here three marine faunas range far northward and 
southward, in parallel lines not far from each other, 
under corresponding climates; but from being sepa- 
rated from each other by impassable barriers, either 
of land or open sea, they are wholly distinct. On the 
other hand, proceeding still further westward from the 
eastern islands of the tropical parts of the Pacific, we 
encounter no impassable barriers, and we have innu- 
merable islands as halting-places, until after travelling 
over a hemisphere we come to the shores of Africa ; 
and over this vast space we meet with no well-defined 
and distinct marine faunas. Although hardly one shell, 
crab or fish is common to the above-named three 
approximate faunas of Eastern and Western America 
and the eastern Pacific islands, yet many fish range 
from the Pacific into the Indian Ocean, and many 
shells are common to the eastern islands of the Pacific 


and the eastern shores of Africa, on almost exactly 
opposite meridians of longitude. 

A third great fact, partly included in the foregoing 
statements, is the affinity of the productions of the 
same continent or sea, though the species themselves 
are distinct at different points and stations. It is 
a law of the widest generality, and every continent 
offers innumerable instances. Nevertheless the natu- 
ralist in travelling, for instance, from north to south 
never fails to be struck by the manner in which suc- 
cessive groups of beings, specifically distinct, yet clearly 
related, replace each other. He hears from closely 
allied, yet distinct kinds of birds, notes nearly similar, 
and sees their nests similarly constructed, but not quite 
alike, with eggs coloured in nearly the same manner. 
The plains near the Straits of Magellan are inhabited 
by one species of Khea (American ostrich), and north- 
ward the plains of La Plata by another species of the 
same genus ; and not by a true ostrich or emeu, like 
those found in Africa and Australia under the same 
latitude. On these same plains of La Plata, we see 
the agouti and bizcacha, animals having nearly the 
same habits as our hares and rabbits and belonging to 
the same order of Kodents, but they plainly display 
an American type of structure. We ascend the lofty 
peaks of the Cordillera and we find an alpine species 
of bizcacha ; we look to the waters, and we do not find 
the beaver or musk-rat, but the coypu and capybara, 
rodents of the American type. Innumerable other in- 
stances could be given. If we look to the islands off 
the American shore, however much they may differ in 
geological structure, the inhabitants, though they may 
be all peculiar species, are essentially American. AW 
may look back to past ages, as shown in the last 
chapter, and we find American types then prevalent on 


the American continent and in the American seas. 
We see in these facts some deep organic bond, prevail- 
ing throughout space and time, over the same areas of 
land and water, and independent of their physical con- 
ditions. The naturalist must feel little curiosity, who 
is not led to inquire what this bond is. 

This bond, on my theory, is simply inheritance, that 
cause which alone, as far as we positively know, pro- 
duces organisms quite like, or, as we see in the case 
of varieties nearly like each other. The dissimilarity of 
the inhabitants of different regions may be attributed 
to modification through natural selection, and in a quite 
subordinate degree to the direct influence of different 
physical conditions. The degree of dissimilarity will de- 
pend on the migration of the more dominant forms of life 
from one region into another having been effected with 
more or less ease, at periods more or less remote ; — on 
the nature and number of the former immigrants ; — 
and on their action and reaction, in their mutual 
struggles for life ; — the relation of organism to organism 
being, as I have already often remarked, the most im- 
portant of all relations. Thus the high importance of 
barriers comes into play by checking migration ; as 
does time for the slow process of modification through 
natural selection. Widely-ranging species, abounding 
in individuals, which have already triumphed over many 
competitors in their own widely-extended homes will 
have the best chance of seizing on new places, when they 
spread into new countries. In their new homes they 
will be exposed to new conditions, and will frequently 
undergo further modification and improvement; and 
thus they will become still further victorious, and Mill 
produce groups of modified descendants. On this prin- 
ciple of inheritance with modification, we can under- 
stand how it is that sections of genera, whole genera, 


and even families are confined to the same areas, as is 
so commonly and notoriously the case. 

I believe, as was remarked in the last chapter, in no 
law of necessary development. As the variability of 
each species is an independent property, and will be 
taken advantage of by natural selection, only so far as 
it profits the individual in its complex struggle for 
life, so the degree of modification in different species 
will be no uniform quantity. If, for instance, a number 
of species, which stand in direct competition with each 
other, migrate in a body into a new and afterwards 
isolated country, they will be little liable to modifica- 
tion ; for neither migration nor isolation in themselves 
can do anything. These principles come into play only 
by bringing organisms into new relations with each other, 
and in a lesser degree with the surrounding physical con- 
ditions. As we have seen in the last chapter that some 
forms have retained nearly the same character from an 
enormously remote geological period, so certain species 
have migrated over vast spaces, and have not become 
greatly modified. 

On these views, it is obvious, that the several species 
of the same genus, though inhabiting the most distant 
quarters of the world, must originally have proceeded 
from the same source, as they have descended from the 
same progenitor. In the case of those species, which 
have undergone during whole geological periods but 
little modification, there is not much difficulty in believ- 
ing that they may have migrated from the same region ; 
for during the vast geographical and climatal changes 
which will have supervened since ancient times, almost 
any amount of migration is possible. But in many other 
cases, in which we have reason to believe that the species 
of a genus have been produced within comparatively 
recent times, there is great difficulty on this head. It 


is also obvious that the individuals of the same species, 
though now inhabiting distant and isolated regions, must 
have proceeded from one spot, where their parents were 
first produced : for, as explained in the last chapter, it 
is incredible that individuals identically the same should 
ever have been produced through natural selection from 
parents specifically distinct. 

We are thus brought to the question which has been 
largely discussed by naturalists, namely, whether species 
have been created at one or more points of the earth's 
surface. Undoubtedly there are very many cases of 
extreme difficulty, in understanding how the same spe- 
cies could possibly have migrated from some one point 
to the several distant and isolated points, where now 
found. Nevertheless the simplicity of the view that 
each species was first produced within a single region 
captivates the mind. He who rejects it, rejects the 
vera causa of ordinary generation with subsequent mi- 
gration, and calls in the agency of a miracle. It is 
universally admitted, that in most cases the area in- 
habited by a species is continuous ; and when a plant 
or animal inhabits two points so distant from each 
other, or with an interval of such a nature, that the 
space could not be easily passed over by migration, the 
fact is given as something remarkable and exceptional. 
The capacity of migrating across the sea is more dis- 
tinctly limited in terrestrial mammals, than perhaps in 
any other organic beings ; and, accordingly, we find no 
inexplicable cases of the same mammal inhabiting dis- 
tant points of the world. No geologist will feel any 
difficulty in such cases as Great Britain having been 
formerly united to Europe, and consequently possessing 
the same quadrupeds. But if the same species can 
be produced at two separate points, why do we not 
find a single mammal common to Europe and Aus- 
tralia or South America? The conditions of life are 


nearly the same, so that a multitude of European animals 
and plants have become naturalised in America and 
Australia ; and some of the aboriginal plants are identi- 
cally the same at these distant points of the northern 
and southern hemispheres ? The answer, as I believe, 
is, that mammals have not been able to migrate, whereas 
some plants, from their varied means of dispersal, have 
migrated across the vast and broken interspace. The 
great and striking influence which barriers of every kind 
have had on distribution, is intelligible only on the view 
that the great majority of species have been produced 
on one side alone, and have not been able to migrate to 
the other side. Some few families, many sub-families, 
very many genera, and a still greater number of sec- 
tions of genera are confined to a single region ; and it 
has been observed by several naturalists, that the most 
natural genera, or those genera in which the species are 
most closely related to each other, are generally local, 
or confined to one area. What a strange anomaly it 
would be, if, when coming one step lower in the series, 
to the individuals of the same species, a directly oppo- 
site rule prevailed ; and species were not local, but had 
been produced in two or more distinct areas ! 

Hence it seems to me, as it has to many other natu- 
ralists, that the view of each species having been pro- 
duced in one area alone, and having subsequently mi- 
grated from that area as far as its powers of migration 
and subsistence under past and present conditions per- 
mitted, is the most probable. Undoubtedly many cases 
occur, in which we cannot explain how the same species 
could have passed from one point to the other. But 
the geographical and climatal changes, which have cer- 
tainly occurred within recent geological times, must 
have interrupted or rendered discontinuous the for- 
merly continuous range of many species. So that 
we are reduced to consider whether the exceptions to 


continuity of range are so numerous and of so grave 
a nature, that we ought to give up the belief, rendered 
probable by general considerations, that each species 
has been produced within one area, and has migrated 
thence as far as it could. It would be hopelessly tedious 
to discuss all the exceptional cases of the same species, 
now living at distant and separated points; nor do I 
for a moment pretend that any explanation could be 
offered of many such cases. But after some preliminary 
remarks, I will discuss a few of the most striking classes 
of facts ; namely, the existence of the same species on 
the summits of distant mountain-ranges, and at distant 
points in the arctic and antarctic regions ; and secondly 
(in the following chapter), the wide distribution of fresh- 
water productions ; and thirdly, the occurrence of the 
same terrestrial species on islands and on the mainland, 
though separated by hundreds of miles of open sea. If 
the existence of the same species at distant and isolated 
points of the earth's surface, can in many instances be 
explained on the view of each species having migrated 
from a single birthplace ; then, considering our ignor- 
ance with respect to former climatal and geographical 
changes and various occasional means of transport, the 
belief that this has been the universal law, seems to me 
incomparably the safest. 

In discussing this subject, we shall be enabled at the 
same time to consider a point equally important for us. 
namely, whether the several distinct species of a genus, 
which on my theory have all descended from a common 
progenitor, can have migrated (undergoing modification 
during some part of their migration) from the area 
inhabited by their progenitor. If it can be shown to 
be almost invariably the case, that a region, of which 
most of its inhabitants are closely related to, or belong- 
to the same genera with the species of a second region, 


lias probably received at some former period immigrants 
from this other region, my theory will be strengthened ; 
for we can clearly understand, on the principle of 
modification, why the inhabitants of a region should be 
related to those of another region, whence it has been 
stocked. A volcanic island, for instance, upheaved and 
formed at the distance of a few hundreds of miles from a 
continent, would probably receive from it in the course 
of time a few colonists, and their descendants, though 
modified, would still be plainly related by inheritance to 
the inhabitants of the continent. Cases of this nature 
are common, and are, as we shall hereafter more fully 
see, inexplicable on the theory of independent creation. 
This view of the relation of species in one region to 
those in another, does not differ much (by substituting 
the word variety for species) from that lately advanced 
in an ingenious paper by Mr. Wallace, in which he con- 
cludes, that "every species has come into existence 
coincident both in space and time with a pre-existing 
closely allied species." And I now know from corre- 
spondence, that this coincidence he attributes to gene- 
ration with modification. 

The previous remarks on " single and multiple centres 
of creation " do not directly bear on another allied 
question, — namely whether all the individuals of the 
same species have descended from a single pair, or 
single hermaphrodite, or whether, as some authors 
suppose, from many individuals simultaneously created. 
With those organic beings which never intercross (if 
such exist), the species, on my theory, must have de- 
scended from a succession of improved varieties, which 
will never have blended with other individuals or varie- 
ties, but will have supplanted each other ; so that, at each 
successive stage of modification and improvement, all 
the individuals of each variety will have descended from 


a single parent. But in the majority of cases, namely, 
with all organisms which habitually unite for each birth, 
or which often intercross, I believe that during the slow 
process of modification the individuals of the species will 
have been kept nearly uniform by intercrossing ; so that 
many individuals will have gone on simultaneously 
changing, and the whole amount of modification will not 
have been due, at each stage, to descent from a single 
parent. To illustrate what I mean : our English race- 
horses differ slightly from the horses of every other 
breed ; but they do not owe their difference and supe- 
riority to descent from any single pair, but to continued 
care in selecting and training many individuals during 
many generations. 

Before discussing the three classes of facts, which 
I have selected as presenting the greatest amount of 
difficulty on the theory of " single centres of creation," 
I must say a few words on the means of dispersal. 

Means of Dispersal. — Sir C. Lyell and other authors 
have ably treated this subject. I can give here only 
the briefest abstract of the more important facts. 
Change of climate must have had a powerful influence 
on migration : a region when its climate was different 
may have been a high road for migration, but now be 
impassable ; I shall, however, presently have to discuss 
this branch of the subject in some detail. Changes 
of level in the land must also have been highly influ- 
ential: a narrow isthmus now separates two marine 
faunas ; submerge it, or let it formerly have been sub- 
merged, and the two faunas will now blend or may 
formerly have blended : where the sea now extends, 
land may at a former period have connected islands or 
possibly even continents together, and thus have allowed 
terrestrial productions to pass from one to the other. 


No geologist will dispute that great mutations of level, 
have occurred within the period of existing organisms. 
Edward Forbes insisted that all the islands in the 
Atlantic must recently have been connected with 
Europe or Africa, and Europe likewise with America. 
Other authors have thus hypothetically bridged over 
every ocean, and have united almost every island to 
some mainland. If indeed the arguments used by 
Forbes are to be trusted, it must be admitted that 
scarcely a single island exists which has not recently 
been united to some continent. This view cuts the 
Gordian knot of the dispersal of the same species to the 
most distant points, and removes many a difficulty : but 
to the best of my judgment we are not authorized in 
admitting such enormous geographical changes within 
the period of existing species. It seems to me that we 
have abundant evidence of great oscillations of level in 
our continents ; but not of such vast changes in their 
position and extension, as to have united them within 
the recent period to each other and to the several inter- 
vening oceanic islands. I freely admit the former ex- 
istence of many islands, now buried beneath the sea, 
which may have served as halting places for plants 
and for many animals during their migration. In the 
coral-producing oceans such sunken islands are now 
marked, as I believe, by rings of coral or atolls standing 
over them. Whenever it is fully admitted, as I believe 
it will some day be, that each species has proceeded 
from a single birthplace, and when in the course of time 
we know something definite about the means of dis- 
tribution, we shall be enabled to speculate with security 
on the former extension of the land. But I do not 
believe that it will ever be proved that within the 
recent period continents which are now quite separate, 
have been continuously, or almost continuously, united 


with each other, and with the many existing oceanic 
islands. Several facts in distribution, — such as the 
great difference in the marine faunas on the opposite 
sides of almost every continent, — the close relation of 
the tertiary inhabitants of several lands and even seas to 
their present inhabitants, — a certain degree of relation 
(as we shall hereafter see) between the distribution of 
mammals and the depth of the sea, — these and other 
such facts seem to me opposed to the admission of such 
prodigious geographical revolutions within the recent 
period, as are necessitated on the view advanced by 
Forbes and admitted by his many followers. The 
nature and relative proportions of the inhabitants of 
oceanic islands likewise seem to me opposed to the 
belief of their former continuity with continents. Nor 
does their almost universally volcanic composition favour 
the admission that they are the wrecks of sunken 
continents ; — if they had originally existed as moun- 
tain-ranges on the land, some at least of the islands 
would have been formed, like other mountain-summits, 
of granite, metamorphic schists, old fossiliferous or 
other such rocks, instead of consisting of mere piles 
of volcanic matter. 

I must now say a few words on what are called acci- 
dental means, but which more properly might be called 
occasional means of distribution. I shall here confine 
myself to plants. In botanical works, this or that plant 
is stated to be ill adapted for wide dissemination ; but 
for transport across the sea, the greater or less facilities 
may be said to be almost wholly unknown. Until I 
tried, with Mr. Berkeley's aid, a few experiments, it 
was not even known how far seeds could resist the inju- 
rious action of sea- water. To my surprise I found that 
out of 87 kinds, 64 germinated after an immersion of 
28 days, and a few survived an immersion of 137 days. 


For convenience sake I chiefly tried small seeds, 
without the capsule or fruit ; and as all of these sank 
in a few days, they could not be floated across wide 
spaces of the sea, whether or not they were injured by 
the salt-water. Afterwards I tried some larger fruits, 
capsules, &c, and some of these floated for a long 
time. It is well known what a difference there is in the 
buoyancy of green and seasoned timber ; and it occurred 
to me that floods might wash down plants or branches, 
and that these might be dried on the banks, and then 
by a fresh rise in the stream be washed into the sea. 
Hence I was led to dry stems and branches of 94 plants 
with ripe fruit, and to place them on sea water. The 
majority sank quickly, but some which whilst green 
floated for a very short time, when dried floated much 
longer ; for instance, ripe hazel-nuts sank immediately, 
but when dried, they floated for 90 days and afterwards 
when planted they germinated ; an asparagus plant 
with ripe berries floated for 23 days, when dried it 
floated for 85 days, and the seeds afterwards germi- 
nated : the ripe seeds of Helosciadium sank in two days, 
when dried they floated for above 90 days, and after- 
wards germinated. Altogether out of the 94 dried 
plants, 18 floated for above 28 days, and some of the 18 
floated for a very much longer period. So that as |^ 
seeds germinated after an immersion of 28 days ; and 
as Jf plants with ripe fruit (but not all the same species 
as in the foregoing experiment) floated, after being dried, 
for above 28 days, as far as we may infer anything from 
these scanty facts, we may conclude that the seeds of 
iVo pl an ts of any country might be floated by sea-cur- 
rents during 28 days, and would retain their power of 
germination. In Johnston's Physical Atlas, the average 
rate of the several Atlantic currents is 33 miles per 
diem (some currents running at the rate of GO miles 


per diem) ; on this average, the seeds of -^^ plants 
belonging to one country might be floated across 924 
miles of sea to another country ; and when stranded, if 
blown to a favourable spot by an inland gale, they would 

Subsequently to my experiments, M. Martens tried 
similar ones, but in a much better manner, for he 
placed the seeds in a box in the actual sea, so that they 
were alternately wet and exposed to the air like really 
floating plants. He tried 98 seeds, mostly different 
from mine ; but he chose many large fruits and likewise 
seeds from plants which live near the sea ; and this 
would have favoured the average length of their flota- 
tion and of their resistance to the injurious action of the 
salt-water. On the other hand he did not previously 
dry the plants or branches with the fruit ; and tins, as 
we have seen, would have caused some of them to have 
floated much longer. The result was that ^f of his 
seeds floated for 42 days, and were then capable of ger- 
mination. But I do not doubt that plants exposed to 
the waves would float for a less time than those pro- 
tected from violent movement as in our experiments. 
Therefore it would perhaps be safer to assume that the 
seeds of about y 1 ^ plants of a flora, after having been 
dried, could be floated across a space of sea 900 miles 
in width, and would then germinate. The fact of the 
larger fruits often floating longer than the small, is in- 
teresting; as plants with large seeds or fruit could 
hardly be transported by any other means ; and Alph. 
de Candolle has shown that such plants generally have 
restricted ranges. 

But seeds may be occasionally transported in another 
manner. Drift timber is thrown up on most islands, 
even on those in the midst of the widest oceans ; and 
the natives of the coral-islands in the Pacific, procure 


stones for their tools, solely from the roots of drifted 
trees, these stones being a valuable royal tax. I find 
on examination, that when irregularly shaped stones 
are embedded in the roots of trees, small parcels of 
earth are very frequently enclosed in their interstices 
and behind them, — so perfectly that not a particle 
could be washed away in the longest transport : out of 
one small portion of earth thus completely enclosed by 
wood in an oak about 50 years old, three dicotyle- 
donous plants germinated : I am certain of the accuracy 
of this observation. Again, I can show that the car- 
casses of birds, when floating on the sea, sometimes 
escape being immediately devoured; and seeds of 
many kinds in the crops of floating birds long retain 
their vitality : peas and vetches, for instance, are killed 
by even a few days' immersion in sea-water ; but some 
taken out of the crop of a pigeon, which had floated on 
artificial salt-water for 30 days, to my surprise nearly all 

Living birds can hardly fail to be highly effective 
agents in the 'transportation of seeds. I could give 
many facts showing how frequently birds of many kinds 
are blown by gales to vast distances across the ocean. 
We may I think safely assume that under such circum- 
stances their rate of flight would often be 35 miles an 
hour ; and some authors have given a far higher esti- 
mate. I have never seen an instance of nutritious 
seeds passing through the intestines of a bird; but 
hard seeds of fruit will pass uninjured through even 
the digestive organs of a turkey. In the course of two 
months, I picked up in my garden 12 kinds of seeds, 
out of the excrement of small birds, and these seemed 
perfect, and some of them, which I tried, germinated. 
But the following fact is more important : the crops of 
birds do not secrete gastric juice, and do not in the 



least injure, as I know by trial, the germination of 
seeds ; now after a bird has found and devoured a large 
supply of food, it is positively asserted that all the grains 
do not pass into the gizzard for 12 or even 18 hours. 
A bird in this interval might easily be blown to the dis- 
tance of 500 miles, and hawks are known to look out 
for tired birds, and the contents of their torn crops 
might thus readily get scattered. Mr. Brent informs me 
that a friend of his had to give up flying carrier-pigeons 
from France to England, as the hawks on the English 
coast destroyed so many on their arrival. Some hawks 
and owls bolt their prey whole, and after an interval 
of from twelve to twenty hours, disgorge pellets, which, 
as I know from experiments made in the Zoological 
Gardens, include seeds capable of germination. Some 
seeds of the oat, wheat, millet, canary, hemp, clover, 
and beet germinated after having been from twelve to 
twenty-one hours in the stomachs of different birds of 
prey ; and two seeds of beet grew after having been 
thus retained for two days and fourteen hours. Fresh- 
water fish, I find, eat seeds of many land and water 
plants : fish are frequently devoured by birds, and thus 
the seeds might be transported from place to place. I 
forced many kinds of seeds into the stomachs of dead 
fish, and then gave their bodies to fishing-eagles, storks, 
and pelicans ; these birds after an interval of many 
hours, either rejected the seeds in pellets or passed 
them in their excrement ; and several of these seeds 
retained their power of germination. Certain seeds, 
however, were always killed by this process. 

Although the beaks and feet of birds are generally 
quite clean, I can show that earth sometimes adheres 
to them : in one instance I removed twenty-two grains 
of dry argillaceous earth from one foot of a partridge, 
and in this earth there was a pebble quite as large as 


the seed of a vetch. Thus seeds might occasionally be 
transported to great distances ; for many facts could be 
given showing that soil almost everywhere is charged 
with seeds. Keflect for a moment on the millions of 
quails which annually cross the Mediterranean ; and can 
we doubt that the earth adhering to their feet would 
sometimes include a few minute seeds? But I shall 
presently have to recur to this subject. 

As icebergs are known to be sometimes loaded with 
earth and stones, and have even carried brushwood, 
bones, and the nest of a land-bird, I can hardly doubt 
that they must occasionally have transported seeds from 
one part to another of the arctic and antarctic regions, 
as suggested by Lyell ; and during the Glacial period 
from one part of the now temperate regions to another. 
In the Azores, from the large number of the species 
of plants common to Europe, in comparison with the 
plants of other oceanic islands nearer to the mainland, 
and (as remarked by Mr. H. C. Watson) from the some- 
what northern character of the flora in comparison with 
the latitude, I suspected that these islands had been 
partly stocked by ice-borne seeds, during the Glacial 
epoch. At my request Sir C. Lyell wrote to M. Hartung 
to inquire whether he had observed erratic boulders 
on these islands, and he answered that he had found 
large fragments of granite and other rocks, which do 
not occur in the archipelago. Hence we may safely 
infer that icebergs formerly landed their rocky burthens 
on the shores of these mid-ocean islands, and it is at 
least possible that they may have brought thither the 
seeds of northern plants. 

Considering that the several above means of trans- 
port, and that several other means, which without 
doubt remain to be discovered, have been in action 
year after year, for centuries and tens of thousands of 


years, it would I think be a marvellous fact if many 
plants had not thus become widely transported. These 
means of transport are sometimes called accidental, but 
this is not strictly correct : the currents of the sea are 
not accidental, nor is the direction of prevalent gales 
of wind. It should be observed that scarcely any 
means of transport would carry seeds for very great 
distances ; for seeds do not retain their vitality when 
exposed for a great length of time to the action of sea- 
water ; nor could they be long carried in the crops or 
intestines of birds. These means, however, would suffice 
for occasional transport across tracts of sea some hun- 
dred miles in breadth, or from island to island, or from 
a continent to a neighbouring island, but not from one 
distant continent to another. The floras of distant 
continents would not by such means become mingled 
in any great degree ; but would remain as distinct 
as we now see them to be. The currents, from their 
course, would never bring seeds from North America 
to Britain, though they might and do bring seeds 
from the West Indies to our western shores, where, 
if not killed by so long an immersion in salt-water, 
they could not endure our climate. Almost every 
year, one or two land-birds are blown across the 
whole Atlantic Ocean, from North America to the 
western shores of Ireland and England ; but seeds 
could be transported by these wanderers only by one 
means, namely, in dirt sticking to their feet, which 
is in itself a rare accident. Even in tins case, how 
small would the chance be of a seed falling on favour- 
able soil, and coming to maturity ! But it would bo 
a great error to argue that because a well-stocked 
island, like Great Britain, has not, as far as is known 
(and it would be very difficult to prove this), received 
within the last few centuries, through occasional means 


of transport, immigrants from Europe or any other 
continent, that a poorly-stocked island, though standing 
more remote from the mainland, would not receive 
colonists by similar means. I do not doubt that out of 
twenty seeds or animals transported to an island, even 
if far less well-stocked than Britain, scarcely more than 
one would be so well fitted to its new home, as to 
become naturalised. But this, as it seems to me, is 
no valid argument against what would be effected by 
occasional means of transport, during the long lapse of 
geological time, whilst an island was being upheaved 
and formed, and before it had become fully stocked 
with inhabitants. On almost bare land, with few or no 
destructive insects or birds living there, nearly every 
seed, which chanced to arrive, would be sure to germi- 
nate and survive. 

Dispersal during the Glacial period. — The identity of 
many plants and animals, on mountain-summits, sepa- 
rated from each other by hundreds of miles of low- 
lands, where the Alpine species could not possibly exist, 
is one of the most striking cases known of the same 
species living at distant points, without the apparent 
possibility of their having migrated from one to the 
other. It is indeed a remarkable fact to see so many 
of the same plants living on the snowy regions of the 
Alps or Pyrenees, and in the extreme northern parts 
of Europe ; but it is far more remarkable, that the 
plants on the White Mountains, in the United States of 
America, are all the same with those of Labrador, and 
nearly all the same, as we hear from Asa Gray, with those 
on the loftiest mountains of Europe. Even as long ago 
as 1747, such facts led Gmelin to conclude that the 
same species must have been independently created at 
several distinct points; and we might have remained 


in this same belief, had not Agassiz and others called 
vivid attention to the Glacial period, which, as we shall 
immediately see, affords a simple explanation of these 
facts. We have evidence of almost every conceivable 
kind, organic and inorganic, that within a very recent 
geological period, central Europe and North America 
suffered under an Arctic climate. The ruins of a house 
burnt by fire do not tell their tale more plainly, than 
do the mountains of Scotland and Wales, with their 
scored flanks, polished surfaces, and perched boulders, 
of the icy streams with which their valleys were lately 
filled. So greatly has the climate of Europe changed, 
that in Northern Italy, gigantic moraines, left by old 
glaciers, are now clothed by the vine and maize. Through- 
out a large part of the United States, erratic boulders, 
and rocks scored by drifted icebergs and coast- ice, plainly 
reveal a former cold period. 

The former influence of the glacial climate on the 
distribution of the inhabitants of Europe, as explained 
with remarkable clearness by Edward Forbes, is sub- 
stantially as follows. But we shall follow the changes 
more readily, by supposing a new glacial period to come 
slowly on, and then pass away, as formerly occurred. As 
the cold came on, and as each more southern zone became 
fitted for arctic beings and ill-fitted for their former 
more temperate inhabitants, the latter would be sup- 
planted and arctic productions would take their places. 
The inhabitants of the more temperate regions would 
at the same time travel southward, unless they were 
stopped by barriers, in which case they would perish. 
The mountains would become covered with snow and 
ice, and their former Alpine inhabitants would descend 
to the plains. By the time that the cold had readied 
its maximum, we should have a uniform arctic fauna 
and flora, covering the central parts of Europe, as far 


south as the Alps and Pyrenees, and even stretching 
into Spain. The now temperate regions of the United 
States would likewise be covered by arctic plants and 
animals, and these would be nearly the same with those 
of Europe ; for the present circumpolar inhabitants, 
which we suppose to have everywhere travelled south- 
ward, are remarkably uniform round the world. We 
may suppose that the Glacial period came on a little 
earlier or later in North America than in Europe, so 
will the southern migration there have been a little 
earlier or later ; but this will make no difference in the 
final result. 

As the warmth returned, the arctic forms would re- 
treat northward, closely followed up in their retreat by 
the productions of the more temperate regions. And as 
the snow melted from the bases of the mountains, the 
arctic forms would seize on the cleared and thawed 
ground, always ascending higher and higher, as the 
warmth increased, whilst their brethren were pursuing 
their northern journey. Hence, when the warmth had 
fully returned, the same arctic species, which had lately 
lived in a body together on the lowlands of the Old and 
New Worlds, would be left isolated on distant mountain- 
summits (having been exterminated on all lesser heights) 
and in the arctic regions of both hemispheres. 

Thus we can understand the identity of many plants 
at points so immensely remote as on the mountains 
of the United States and of Europe. We can thus 
also understand the fact that the Alpine plants of 
each mountain-range are more especially related to 
the arctic forms living due, north or nearly due north 
of them : for the migration as the cold came on, and the 
re-migration on the returning warmth, will generally 
have been due south and north. The Alpine plants, for 
example, of Scotland, as remarked by Mr. H. C. AVatson, 


and those of the Pyrenees, as remarked by Kamond, are 
more especially allied to the plants of northern Scan- 
dinavia ; those of the United States to Labrador ; those 
of the mountains of Siberia to the arctic regions of that 
country. These views, grounded as they are on the 
perfectly well-ascertained occurrence of a former Glacial 
period, seem to me to explain in so satisfactory a 
manner the present distribution of the Alpine and 
Arctic productions of Europe and America, that when in 
other regions we find the same species on distant moun- 
tain-summits, we may almost conclude without other 
evidence, that a colder climate permitted their former 
migration across the low intervening tracts, since be- 
come too warm for their existence. 

If the climate, since the Glacial period, has ever been 
in any degree warmer than at present (as some geo- 
logists in the United States believe to have been the 
case, chiefly from the distribution of the fossil Gnatho- 
don), then the arctic and temperate productions will at 
a very late period have marched a little further north, 
and subsequently have retreated to their present homes ; 
but I have met with no satisfactory evidence with respect 
to this intercalated slightly warmer period, since the 
Glacial period. 

The arctic forms, during their long southern migra- 
tion and re-migration northward, will have been exposed 
to nearly the same climate, and, as is especially to be 
noticed, they will have kept in a body together ; con- 
sequently their mutual relations will not have been 
much disturbed, and, in accordance with the principles 
inculcated in this volume, they will not have been liable 
to much modification. But with our Alpine productions, 
left isolated from the moment of the returning warmth, 
first at the bases and ultimately on the summits of 
the mountains, the case will have been somewhat dif- 


ferent ; for it is not likely that all the same arctic spe- 
cies will have been left on mountain ranges distant from 
each other, and have survived there ever since ; they 
will, also, in all probability have become mingled with 
ancient Alpine species, which must have existed on 
the mountains before the commencement of the Glacial 
epoch, and which during its coldest period will have 
been temporarily driven down to the plains ; they will, 
also, have been exposed to somewhat different climatal 
influences. Their mutual relations will thus have been 
in some degree disturbed ; consequently they will have 
been liable to modification ; and this we find has been 
the case ; for if we compare the present Alpine plants 
and animals of the several great European mountain- 
ranges, though very many of the species are identically 
the same, some present varieties, some are ranked as 
doubtful forms, and some few are distinct yet closely 
allied or representative species. 

In illustrating what, as I believe, actually took place 
during the Glacial period, I assumed that at its com- 
mencement the arctic productions were as uniform 
round the polar regions as they are at the present day. 
But the foregoing remarks on distribution apply not 
only to strictly arctic forms, but also to many sub-arctic 
and to some few northern temperate forms, for some of 
these are the same on the lower mountains and on the 
plains of North America and Europe ; and it may be 
reasonably asked how I account for the necessary de- 
gree of uniformity of the sub-arctic and northern tem- 
perate forms round the world, at the commencement of 
the Glacial period. At the present day, the sub-arctic 
and northern temperate productions of the Old and 
New Worlds are separated from each other by the 
Atlantic Ocean and by the extreme northern part of 
the Pacific. During the Glacial period, when the in- 

R 3 


habitants of the Old and New Worlds lived further 
southwards than at present, they must have been still 
more completely separated by wider spaces of ocean. 
I believe the above difficulty may be surmounted by 
looking to still earlier changes of climate of an opposite 
nature. We have good reason to believe that during 
the newer Pliocene period, before the Glacial epoch, 
and whilst the majority of the inhabitants of the world 
were specifically the same as now, the climate was 
warmer than at the present day. Hence we may sup- 
pose that the organisms now living under the climate 
of latitude 60°, during the Pliocene period lived further 
north under the Polar Circle, in latitude 66°-67° ; and 
that the strictly arctic productions then lived on the 
broken land still nearer to the pole. Now if we look at 
a globe, we shall see that under the Polar Circle there 
is almost continuous land from western Europe, through 
Siberia, to eastern America. And to tins continuity of 
the circumpolar land, and to the consequent freedom 
for intermigration under a more favourable climate, I 
attribute the necessary amount of uniformity in the 
sub-arctic and northern temperate productions of the 
Old and New Worlds, at a period anterior to the Glacial 

Believing, from reasons before alluded to, that our 
continents have long remained in nearly the same rela- 
tive position, though subjected to large, but partial 
oscillations of level, I am strongly inclined to extend 
the above view, and to infer that during some earlier 
and still warmer period, such as the older Pliocene 
period, a large number of the same plants and animals 
inhabited the almost continuous circumpolar land ; and 
that these plants and animals, both in the Old and 
New Worlds, began slowly to migrate southwards as 
the climate became less warm, long before the com- 


mencernent of tlie Glacial period. We now ' see, as I 
believe, their descendants, mostly in a modified con- 
dition, in the central parts of Europe and the United 
States. On this view we can understand the relation- 
ship, with very little identity, between the productions 
of North America and Europe, — a relationship which is 
most remarkable, considering the distance of the two 
areas, and their separation by the Atlantic Ocean. We 
can further understand the singular fact remarked on 
by several observers, that the productions of Europe 
and America during the later tertiary stages were more 
closely related to each other than they are at the pre- 
sent time ; for during these warmer periods the northern 
parts of the Old and New Worlds will have been almost 
continuously united by land, serving as a bridge, since 
rendered impassable by cold, for the inter-migration of 
their inhabitants. 

During the slowly decreasing warmth of the Pliocene 
period, as soon as the species in common, which inhabited 
the New and Old Worlds, migrated south of the Polar 
Circle, they must have been completely cut off from each 
other. This separation, as far as the more temperate pro- 
ductions are concerned, took place long ages ago. And 
as the plants and animals migrated southward, they will 
have become mingled in the one great region with the 
native American productions, and have had to compete 
with them ; and in the other great region, with those 
of the Old World. Consequently we have here every- 
thing favourable for much modification, — for far more 
modification than with the Alpine productions, left 
isolated, within a much more recent period, on the 
several mountain-ranges and on the arctic lands of the 
two Worlds. Hence it has come, that when we compare 
the now living productions of the temperate regions of 
the New and Old Worlds, we find very few identical 


species (though Asa Gray has lately shown that more 
plants are identical than was formerly supposed), but 
we find in every great class many forms, which some 
naturalists rank as geographical races, and others as dis- 
tinct species ; and a host of closely allied or represen- 
tative forms which are ranked by all naturalists as 
specifically distinct. 

As on the land, so in the waters of the sea, a slow 
southern migration of a marine fauna, which during 
the Pliocene or even a somewhat earlier period, was 
nearly uniform along the continuous shores of the Polar 
Circle, will account, on the theory of modification, for 
many closely allied forms now living in areas completely 
sundered. Thus, I think, we can understand the pre- 
sence of many existing and tertiary representative forms 
on the eastern and western shores of temperate North 
America ; and the still more striking case of many 
closely allied crustaceans (as described in Dana's ad- 
mirable work), of some fish and other marine animals, in 
the Mediterranean and in the seas of Japan, — areas 
now separated by a continent and by nearly a hemi- 
sphere of equatorial ocean. 

These cases of relationship, without identity, of the 
inhabitants of seas now disjoined, and likewise of the 
past and present inhabitants of the temperate lands of 
North America and Europe, are inexplicable on the 
theory of creation. We cannot say that they have 
been created alike, in correspondence with the nearly 
similar physical conditions of the areas ; for if we com- 
pare, for instance, certain parts of South America with 
the southern continents of the Old World, we see 
countries closely corresponding in all their physical 
conditions, but with their inhabitants utterly dissimilar. 

But we must return to our more immediate subject, 
the Glacial period. I am convinced that Forbes's view 


may be largely extended. In Europe we have the 
plainest evidence of the cold period, from the western 
shores of Britain to the Oural range, and southward to 
the Pyrenees. We may infer, from the frozen mammals 
and nature of the mountain vegetation, that Siberia was 
similarly affected. Along the Himalaya, at points 900 
miles apart, glaciers have left the marks of their former 
low descent; and in Sikkim, Dr. Hooker saw maize 
growing on gigantic ancient moraines. South of the 
equator, we have some direct evidence of former glacial 
action in New Zealand ; and the same plants, found on 
widely separated mountains in this island, tell the same 
story. If one account which has been published can be 
trusted, we have direct evidence of glacial action in the 
south-eastern corner of Australia. 

Looking to America ; in the northern half, ice-borne 
fragments of rock have been observed on the eastern 
side as far south as lat. 36°-37°, and on the shores of 
the Pacific, where the climate is now so different, as 
far south as lat. 46° ; erratic boulders have, also, been 
noticed on the Kocky Mountains. In the Cordillera 
of Equatorial South America, glaciers once extended 
far below their present level. In central Chile I was 
astonished at the structure of a vast mound of detritus, 
about 800 feet in height, crossing a valley of the 
Andes ; and this I now feel convinced was a gigantic 
moraine, left far below any existing glacier. Further 
south on both sides of the continent, from lat. 41° to 
the southernmost extremity, we have the clearest 
evidence of former glacial action, in huge boulders 
transported far from their parent source. 

We do not know that the Glacial epoch was strictly 
simultaneous at these several far distant points on oppo- 
site sides of the world. But we have good evidence in 
almost every case, that the epoch was included within 


the latest geological period. We have, also, excellent 
evidence, that it endured for an enormous time, as 
measured by years, at each point. The cold may have 
come on, or have ceased, earlier at one point of the 
globe than at another, but seeing that it endured for 
long at each, and that it was contemporaneous in a 
geological sense, it seems to me probable that it was, 
during a part at least of the period, actually simulta- 
neous throughout the world. Without some distinct 
evidence to the contrary, we may at least admit as 
probable that the glacial action was simultaneous on 
the eastern and western sides of North America, in the 
Cordillera under the equator and under the warmer 
temperate zones, and on both sides of the southern 
extremity of the continent. If this be admitted, it is 
difficult to avoid believing that the temperature of 
the whole world was at this period simultaneously 
cooler. But it would suffice for my purpose, if the 
temperature was at the same time lower along certain 
broad belts of longitude. 

On this view of the whole world, or at least of broad 
longitudinal belts, having been simultaneously colder 
from pole to pole, much light can be thrown on the 
present distribution of identical and allied species. In 
America, Dr. Hooker has shown that between forty and 
fifty of the flowering plants of Tierra del Fuego, forming 
no inconsiderable part of its scanty flora, are common to 
Europe, enormously remote as these two points are ; and 
there are many closely allied species. On the lofty 
mountains of equatorial America a host of peculiar 
species belonging to European genera occur. On the 
highest mountains of Brazil, some few European genera 
were found by Gardner, which do not exist in the wide 
intervening hot countries. So on the Silla of Caraccas 
the illustrious Humboldt long ago found species belong- 


ing to genera characteristic of the Cordillera. On the 
mountains of Abyssinia, several European forms and 
some few representatives of the peculiar flora of the 
Cape of Good Hope occur. At the Cape of Good Hope 
a very few European species, believed not to have been 
introduced by man, and on the mountains, some few 
representative European forms are found, which have 
not been discovered in the intertropical parts of Africa. 
On the Himalaya, and on the isolated mountain-ranges 
of the peninsula of India, on the heights of Ceylon, and 
on the volcanic cones of Java, many plants occur, either 
identically the same or representing each other, and at 
the same time representing plants of Europe, not found 
in the intervening hot lowlands. A list of the genera 
collected on the loftier peaks of Java raises a picture 
of a collection made on a hill in Europe ! Still more 
striking is the fact that southern Australian forms are 
clearly represented by plants growing on the summits 
of the mountains of Borneo. Some of these Australian 
forms, as I hear from Dr. Hooker, extend along the 
heights of the peninsula of Malacca, and are thinly 
scattered, on the one hand over India and on the other 
as far north as Japan. 

On the southern mountains of Australia, Dr. F. 
Mtiller has discovered several European species ; other 
species, not introduced by man, occur on the lowlands ; 
and a long list can be given, as I am informed by Dr. 
Hooker, of European genera, found in Australia, but 
not in the intermediate torrid regions. In the admir- 
able ' Introduction to the Flora of New Zealand,' by 
Dr. Hooker, analogous and striking facts are given in 
regard to the plants of that large island. Hence we see 
that throughout the world, the plants growing on the 
more lofty mountains, and on the temperate lowlands 
of the northern and southern hemispheres, are sometimes 


identically the same ; but they are much oftener speci- 
fically distinct, though related to each other in a most 
remarkable manner. 

This brief abstract applies to plants alone : some 
strictly analogous facts could be given on the distribu- 
tion of terrestrial animals. In marine productions, 
similar cases occur ; as an example, I may quote a 
remark by the highest authority, Prof. Dana, that " it 
is certainly a wonderful fact that New Zealand should 
have a closer resemblance in its Crustacea to Great 
Britain, its antipode, than to any other part of the 
world." Sir J. Eichardson, also, speaks of the re- 
appearance on the shores of New Zealand, Tasmania, 
&c, of northern forms of fish. Dr. Hooker informs 
me that twenty-five species of Algae are common to 
New Zealand and to Europe, but have not been found 
in the intermediate tropical seas. 

It should be observed that the northern species and 
forms found in the southern parts of the southern hemi- 
sphere, and on the mountain-ranges of the intertropical 
regions, are not arctic, but belong to the northern tem- 
perate zones. As Mr. H. C. Watson has recently re- 
marked, " In receding from polar towards equatorial 
latitudes, the Alpine or mountain floras really become 
less and less arctic." Many of the forms living on the 
mountains of the warmer regions of the earth and in 
the southern hemisphere are of doubtful value, being 
ranked by some naturalists as specifically distinct, by 
others as varieties ; but some are certainly identical, 
and many, though closely related to northern forms, 
must be ranked as distinct species. 

Now let us see what light can be thrown on the fore- 
going facts, on the belief, supported as it is by a large 
body of geological evidence, that the whole world, or a 
large part of it, was during the Glacial period siniulta- 


neously much colder than at present. The Glacial 
period, as measured by years, must have been very 
long; and when we remember over what vast spaces 
some naturalised plants and animals have spread within 
a few centuries, this period will have been ample for 
any amount of migration. As the cold came slowly on, 
all the tropical plants and other productions will have 
retreated from both sides towards the equator, followed 
in the rear by the temperate productions, and these by 
the arctic ; but with the latter we are not now con- 
cerned. The tropical plants probably suffered much 
extinction ; how much no one can say ; perhaps for- 
merly the tropics supported as many species as we see 
at the present day crowded together at the Cape of 
Good Hope, and in parts of temperate Australia. As 
we know that many tropical plants and animals can 
withstand a considerable amount of cold, many might 
have escaped extermination during a moderate fall of 
temperature, more especially by escaping into the 
warmest spots. But the great fact to bear in mind is, 
that all tropical productions will have suffered to a cer- 
tain extent. On the other hand, the temperate pro- 
ductions, after migrating nearer to the equator, though 
they will have been placed under somewhat new con- 
ditions, will have suffered less. And it is certain that 
many temperate plants, if protected from the inroads 
of competitors, can withstand a much warmer climate 
than their own. Hence, it seems to me possible, 
bearing in mind that the tropical productions were 
in a suffering state and could not have presented a 
firm front against intruders, that a certain number of 
the more vigorous and dominant temperate forms might 
have penetrated the native ranks and have reached or 
even crossed the equator. The invasion would, of course, 
have been greatly favoured by high land, and perhaps 


by a dry climate ; for Dr. Falconer informs me that it 
is the damp with the heat of the tropics which is so 
destrnctive to perennial plants from a temperate cli- 
mate. On the other hand, the most humid and hottest 
districts will have afforded an asylum to the tropical 
natives. The mountain-ranges north-west of the Hima- 
laya, and the long line of the Cordillera, seem to have 
afforded two great lines of invasion : and it is a striking 
fact, lately communicated to me by Dr. Hooker, that all 
the flowering plants, about forty-six in number, common 
to Tierra del Fuego and to Europe still exist in North 
America, which must have lain on the line of march. 
But I do not doubt that some temperate productions 
entered and crossed even the lowlands of the tropics at 
the period when the cold was most intense, — when 
arctic forms had migrated some twenty-five degrees 
of latitude from their native country and covered the 
land at the foot of the Pyrenees. At this period of ex- 
treme cold, I believe that the climate under the equator 
at the level of the sea was about the same with that now 
felt there at the height of six or seven thousand feet. 
During this the coldest period, I suppose that large 
spaces of the tropical lowlands were clothed with a 
mingled tropical and temperate vegetation, like that 
now growing with strange luxuriance at the base of the 
Himalaya, as graphically described by Hooker. 

Thus, as I believe, a considerable number of plants, a 
few terrestrial animals, and some marine productions, 
migrated during the Glacial period from the northern 
and southern temperate zones into the intertropical re- 
gions, and some even crossed the equator. As the warmth 
returned, these temperate forms would naturally ascend 
the higher mountains, being exterminated on the low- 
lands ; those which had not reached the equator, would 
re-migrate northward or southward towards their former 


homes ; but the forms, chiefly northern, which had 
crossed the equator, would travel still further from their 
homes into the more temperate latitudes of the opposite 
hemisphere. Although we have reason to believe from 
geological evidence that the whole body of arctic shells 
underwent scarcely any modification during their long 
southern migration and re-migration northward, the case 
may have been wholly different with those intruding 
forms which settled themselves on the intertropical 
mountains, and in the southern hemisphere. These 
being surrounded by strangers will have had to compete 
with many new forms of life ; and it is probable that 
selected modifications in their structure, habits, and con- 
stitutions will have profited them. Thus many of these 
wanderers, though still plainly related by inheritance to 
their brethren of the northern or southern hemispheres, 
now exist in their new homes as well-marked varieties 
or as distinct species. 

It is a remarkable fact, strongly insisted on by 
Hooker in regard to America, and by Alph. de Candolle 
in regard to Australia, that many more identical plants 
and allied forms have apparently migrated from the 
north to the south, than in a reversed direction. We 
see, however, a few southern vegetable forms on the 
mountains of Borneo and Abyssinia. I suspect that 
this preponderant migration from north to south is due 
to the greater extent of land in the north, and to the 
northern forms having existed in their own homes in 
greater numbers, and having consequently been ad- 
vanced through natural selection and competition to a 
higher stage of perfection or dominating power, than the 
southern forms. And thus, when they became com- 
mingled during the Glacial period, the northern forms 
were enabled to beat the less powerful southern forms. 
Just in the same manner as we see at the present day, 


that very many European productions cover the ground 
in La Plata, and in a lesser degree in Australia, and 
have to a certain extent beaten the natives ; whereas 
extremely few southern forms have become naturalised 
in any part of Europe, though hides, wool, and other 
objects likely to carry seeds have been largely im- 
ported into Europe during the last two or three cen- 
turies from La Plata, and during the last thirty or forty 
years from Australia. Something of the same kind 
must have occurred on the intertropical mountains : no 
doubt before the Glacial period they were stocked with 
endemic Alpine forms ; but these have almost every- 
where largely yielded to the more dominant forms, 
generated in the larger areas and more efficient work- 
shops of the north. In many islands the native pro- 
ductions are nearly equalled or even outnumbered by 
the naturalised ; and if the natives have not been actu- 
ally exterminated, their numbers have been greatly 
reduced, and this is the first stage towards extinction. 
A mountain is an island on the land; and the inter- 
tropical mountains before the Glacial period must have 
been completely isolated ; and I believe that the pro- 
ductions of these islands on the land yielded to those 
produced within the larger areas of the north, just in 
the same way as the productions of real islands have 
everywhere lately yielded to continental forms, natu- 
ralised by man's agency. 

I am far from supposing that all difficulties are re- 
moved on the view here given in regard to the range 
and affinities of the allied species winch live in the 
northern and southern temperate zones and on the 
mountains of the intertropical regions. Veiy many 
difficulties remain to be solved. I do not pretend to 
indicate the exact lines and means of migration, or the 
reason why certain species and not others have migrated ; 


why certain species have been modified and have given 
rise to new groups of forms, and others have remained 
unaltered. We cannot hope to explain such facts, 
until we can say why one species and not another be- 
comes naturalised by man's agency in a foreign land ; 
why one ranges twice or thrice as far, and is twice or 
thrice as common, as another species within their own 

I have said that many difficulties remain to be solved : 
some of the most remarkable are stated with admirable 
clearness by Dr. Hooker in his botanical works on the 
antarctic regions. These cannot be here discussed. I 
will only say that as far as regards the occurrence of 
identical species at points so enormously remote as 
Kerguelen Land, New Zealand, and Fuegia, I believe 
that towards the close of the Glacial period, icebergs, 
as suggested by Lyell, have been largely concerned in 
their dispersal. But the existence of several quite 
distinct species, belonging to genera exclusively confined 
to the south, at these and other distant points of the 
southern hemisphere, is, on my theory of descent with 
modification, a far more remarkable case of difficulty. 
For some of these species are so distinct, that we cannot 
suppose that there has been time since the commence- 
ment of the Glacial period for their migration, and 
for their subsequent modification to the necessary 
degree. The facts seem to me to indicate that pe- 
culiar and very distinct species have migrated in radi 
a ting lines from some common centre ; and I am in- 
clined to look in the southern, as in the northern hemi- 
sphere, to a former and warmer period, before the com- 
mencement of the Glacial period, when the antarctic 
lands, now covered with ice, supported a highly peculiar 
and isolated flora. I suspect that before tins flora was 
exterminated by the Glacial ej>och, a few forms were 


widely dispersed to various points of the southern 
hemisphere by occasional means of transport, and by 
the aid, as halting-places, of existing and now sunken 
islands, and perhaps at the commencement of the 
Glacial period, by icebergs. By these means, as I be- 
lieve, the southern shores of America, Australia, New 
Zealand have become slightly tinted by the same pecu- 
liar forms of vegetable life. 

Sir C. Lyell in a striking passage has speculated, in 
language almost identical with mine, on the effects of 
great alternations of climate on geographical distri- 
bution. I believe that the world has recently felt one 
of his great cycles of change ; and that on this view, 
combined with modification through natural selection, 
a multitude of facts in the present distribution both 
of the same and of allied forms of life can be ex- 
plained. The living waters may be said to have flowed 
during one short period from the north and from the 
south, and to have crossed at the equator; but to 
have flowed with greater force from the north so as to 
have freely inundated the south. As the tide leaves 
its drift in horizontal lines, though rising higher on 
the shores where the tide rises highest, so have the 
living waters left their living drift on our mountain- 
summits, in a line gently rising from the arctic low- 
lands to a great height under the equator. The various 
beings thus left stranded may be compared with savage 
races of man, driven up and surviving in the mountain- 
fastnesses of almost every land, which serve as a record, 
full of interest to us, of the former inhabitants of the 
surrounding lowlands. 



Geographical Distribution — continued. 

Distribution of fresh-water productions — On the inhabitants of 
oceanic islands — Absence of Batrachians and of terrestrial Mam- 
mals — On the relation of the inhabitants of islands to those of 
the nearest mainland — On colonisation from the nearest source 
with subsequent modification — Summary of the last and pre- 
sent chapters. 

As lakes and river-systems are separated from each 
other by barriers of land, it might have been thought 
that fresh-water productions would not have ranged 
widely within the same country, and as the sea is ap- 
parently a still more impassable barrier, that they 
never would have extended to distant countries. But 
the case is exactly the reverse. Not only have many 
fresh-water species, belonging to quite different classes, 
an enormous range, but allied species prevail in a 
remarkable manner throughout the world. I well re- 
member, when first collecting in the fresh waters of 
Brazil, feeling much surprise at the similarity of the 
fresh-water insects, shells, &c, and at the dissimilarity 
of the surrounding terrestrial beings, compared with 
those of Britain. 

But tins power in fresh-water productions of ranging 
widely, though so unexpected, can, I think, in most 
cases be explained by their having become fitted, in 
a manner highly useful to them, for short and frequent 
migrations from pond to pond, or from stream to 
stream ; and liability to wide dispersal would follow 
from this capacity as an almost necessary consequence. 
We can here consider only a few cases. In regard to 


fish, I believe that the same species never occur in 
the fresh waters of distant continents. But on the 
same continent the species often range widely and 
almost capriciously; for two river-systems will have 
some fish in common and some different. A few facts 
seem to favour the possibility of their occasional trans- 
port by accidental means ; like that of the live fish not 
rarely dropped by whirlwinds in India, and the vitality 
of their ova when removed from the water. But I am 
inclined to attribute the dispersal of fresh-water fish 
mainly to slight changes within the recent period in 
the level of the land, having caused rivers to flow into 
each other. Instances, also, could be given of this 
having occurred during floods, without any change of 
level. We have evidence in the loess of the Rhine of 
considerable changes of level in the land within a very 
recent geological period, and when the surface was 
peopled by existing land and fresh-water shells. The 
wide difference of the fish on opposite sides of con- 
tinuous mountain-ranges, which from an early period 
must have parted river-systems and completely pre- 
vented their inosculation, seems to lead to this same 
conclusion. With respect to allied fresh-water fish 
occurring at very distant points of the world, no doubt 
there are many cases winch cannot at present be ex- 
plained: but some fresh-water fish belong to very 
ancient forms, and in such cases there will have been 
ample time for great geographical changes, and con- 
sequently time and means for much migration. In 
the second place, salt-water fish can with care be slowly 
accustomed to live in fresh water; and, according to 
Valenciennes, there is hardly a single group of fishes 
confined exclusively to fresh water, so that we may 
imagine that a marine member of a fresh-water group 
might travel far along the shores of the sea, and subse- 


quently become modified and adapted to the fresh 
waters of a distant land. 

Some species of fresh-water shells have a very wide 
range, and allied species, which, on my theory, are de- 
scended from a common parent and mnst have proceeded 
from a single source, prevail throughout the world. 
Their distribution at first perplexed me much, as their 
ova are not likely to be transported by birds, and they 
are immediately killed by sea water, as are the adults. 
I could not even understand how some naturalised 
species have rapidly spread throughout the same 
country. But two facts, which I have observed — and 
no doubt many others remain to be observed — throw 
some light on tins subject. When a duck suddenly 
emerges from a pond covered with duck-weed, I have 
twice seen these little plants adhering to its back ; and 
it has happened to me, in removing a little duck- 
weed from one aquarium to another, that I have quite 
unintentionally stocked the one with fresh-water shells 
from the other. But another agency is perhaps more 
effectual : I suspended a duck's feet, which might 
represent those of a bird sleeping in a natural pond, 
in an aquarium, where many ova of fresh- water shells 
were hatching; and I found that numbers of the ex- 
tremely minute and just hatched shells crawled on the 
feet, and clung to them so firmly that when taken out 
of the water they could not be jarred off, though at 
a somewhat more advanced age they would voluntarily 
drop off. These just hatched molluscs, though aquatic 
in their nature, survived on the duck's feet, in damp 
air, from twelve to twenty hours ; and in this length of 
time a duck or heron might fly at least six or seven 
hundred miles, and would be sure to alight on a pool 
or rivulet, if blown across sea to an oceanic island or 
to any other distant point. Sir Charles Lyell also 


informs me that a Dyticus has been caught with an 
Ancylus (a fresh-water shell like a limpet) firmly ad- 
hering to it ; and a water-beetle of the same family, a 
Colymbetes, once flew on board the ' Beagle,' when 
forty-five miles distant from the nearest land : how 
much farther it might have flown with a favouring gale 
no one can tell. 

With respect to plants, it has long been known what 
enormous ranges many fresh-water and even marsh- 
species have, both over continents and to the most 
remote oceanic islands. This is strikingly shown, as 
remarked by Alph. de Candolle, in large groups of 
terrestrial plants, winch have only a very few aquatic 
members ; for these latter seem immediately to acquire, 
as if in consequence, a very wide range. I think favour- 
able means of dispersal explain this fact. I have before 
mentioned that earth occasionally, though rarely, ad- 
heres in some quantity to the feet and beaks of birds/ 
Wading birds, which frequent the muddy edges of 
ponds, if suddenly flushed, would be the most likely to 
have muddy feet. Birds of this order I can show are 
the greatest wanderers, and are occasionally found on 
the most remote and barren islands in the open ocean ; 
they would not be likely to alight on the surface of the 
sea, so that the dirt would not be washed off their feet ; 
when making land, they would be sure to fly to their 
natural fresh-water haunts. I do not believe that 
botanists are aware how charged the mud of ponds is 
with seeds : I have tried several little experiments, but 
will here give only the most striking case : I took in 
February three table-spoonfuls of mud from three dif- 
ferent points, beneath water, on the edge of a little 
pond; this mud when dry weighed only <i;, : ounces; I 
kept it covered up in my study for six months, pulling 
up and counting each plant as it grew ; the plants were 


of many kinds, and were altogether 537 in number ; 
and yet the viscid mud was all contained in a breakfast 
cup ! Considering these facts, I think it would be an 
inexplicable circumstance if water-birds did not trans- 
port the seeds of fresh-water plants to vast distances, 
and if consequently the range of these plants was not 
very great. The same agency may have come into 
play with the eggs of some of the smaller fresh-water 

Other and unknown agencies probably have also 
played a part. I have stated that fresh-water fish eat 
some kinds of seeds, though they reject many other 
kinds after having swallowed them; even small fish 
swallow seeds of moderate size, as of the yellow water- 
lily and Potamogeton. Herons and other birds, century 
after century, have gone on daily devouring fish ; they 
then take flight and go to other waters, or are blown 
across the sea ; and we have seen that seeds retain their 
power of germination, when rejected in pellets or in 
excrement, many hours afterwards. When I saw the 
great size of the seeds of that fine water-lily, the 
Nelumbium, and remembered Alph. de Candolle's re- 
marks on this plant, I thought that its distribution 
must remain quite inexplicable ; but Audubon states 
that he found the seeds of the great southern water- 
lily (probably, according to Dr. Hooker, the Nelumbium 
luteum) in a heron's stomach ; although I do not know 
the fact, yet analogy makes me believe that a heron 
flying to another pond and getting a hearty meal of 
fish, would probably reject from its stomach a pellet 
containing the seeds of the Nelumbium midigested; 
or the seeds might be dropped by the bird whilst 
feeding its young, in the same way as fish are known 
sometimes to be dropped. 

In considering these several means of distribution, 



it should be remembered that when a pond or stream 
is first formed, for instance, on a rising islet, it will be 
unoccupied ; and a single seed or egg will have a good 
chance of succeeding. Although there will always be a 
struggle for life between the individuals of the species, 
however few, already occupying any pond, yet as the 
number of kinds is small, compared with those on the 
land, the competition will probably be less severe 
between aquatic than between terrestrial species ; con- 
sequently an intruder from the waters of a foreign 
country, would have a better chance of seizing on a 
place, than in the case of terrestrial colonists. We 
should, also, remember that some, perhaps many, fresh- 
water productions are low in the scale of nature, and 
that we have reason to believe that such low beings 
change or become modified less quickly than the high ; 
and tins will give longer time than the average for the 
migration of the same aquatic species. We should not 
forget the probability of many species having formerly 
ranged as continuously as fresh- water productions ever 
can range, over immense areas, and having subsequently 
become extinct in intermediate regions. But the wide 
distribution of fresh- water plants and of the Lower 
animals, whether retaining the same identical form 
or in some degree modified, I believe mainly depends 
on the wide dispersal of their seeds and eggs by animals, 
more especially by fresh-water birds, which have large 
powers of flight, and naturally travel from one to 
another and often distant piece of water. Nature, like 
a careful gardener, thus takes her seeds from a bed of 
a particular nature, and drops them in another equally 
well fitted for them. 

On the Inhabitants of Oceanic Islands. — We now 
come to the last of the three classes of facts, which I 


have selected as presenting the greatest amount of 
difficulty, on the view that all the individuals both of 
the same and of allied species have descended from a 
single parent ; and therefore have all proceeded from a 
common birthplace, notwithstanding that in the course 
of time they have come to inhabit distant points of the 
globe. I have already stated that I cannot honestly 
admit Forbes's view on continental extensions, which, 
if legitimately followed out, would lead to the belief 
that within the recent period all existing islands have 
been nearly or quite joined to some continent. This view 
would remove many difficulties, but it would not, I 
think, explain all the facts in regard to insular produc- 
tions. In the following remarks I shall not confine 
myself to the mere question of dispersal; but shall 
consider some other facts, which bear on the truth of 
the two theories of independent creation and of descent 
with modification. 

The species of all kinds which inhabit oceanic islands 
are few in number compared with those on equal con- 
tinental areas : Alph. de Candolle admits this for plants, 
and Wollaston for insects. If we look to the large 
size and varied stations of New Zealand, extending over 
780 miles of latitude, and compare its flowering plants, 
only 750 in number, with those on an equal area at 
the Cape of Good Hope or in Australia, we must, I 
think, admit that something quite independently of 
any difference in physical conditions has caused so great 
a difference in number. Even the uniform county of 
Cambridge has 847 plants, and the little island of 
Anglesea 764, but a few ferns and a few introduced 
plants are included in these numbers, and the com- 
parison in some other respects is not quite fair. We 
have evidence that the barren island of Ascension 
aboriginally possessed under half-a-dozen flowering 


plants ; yet many have become naturalised on it, 
as they have on New Zealand and on every other 
oceanic island which can be named. In St. Helena 
there is reason to believe that the naturalised plants 
and animals have nearly or quite exterminated many 
native productions. He who admits the doctrine of 
the creation of each separate species, will have to 
admit, that a sufficient number of the best adapted 
plants and animals have not been created on oceanic 
islands ; for man has unintentionally stocked them from 
various sources far more fully and perfectly than has 

Although in oceanic islands the number of kinds 
of inhabitants is scanty, the proportion of endemic 
species (». e. those found nowhere else in the world) 
is often extremely large. If we compare, for instance, 
the number of the endemic land-shells in Madeira, or 
of the endemic birds in the Galapagos Archipelago, with 
the number found on any continent, and then compare 
the area of the islands with that of the continent, we 
shall see that this is true. This fact might have been 
expected on my theory, for, as already explained, spe- 
cies occasionally arriving after long intervals in a new 
and isolated district, and having to compete with new 
associates, will be eminently liable to modification, and 
will often produce groups of modified descendants. But 
it by no means follows, that, because in an island nearly 
all the species of one class are peculiar, those of another 
class, or of another section of the same class, are pecu- 
liar ; and this difference seems to depend on the species 
which do not become modified having immigrated with 
facility and in a body, so that their mutual relations 
have not been much disturbed. Thus in the Galapagos 
Islands nearly every land-bird, but only two out of the 
eleven marine birds, are peculiar ; and it is obvious that 


marine birds could arrive at these islands more easily than 
land-birds. Bermuda, on the other hand, which lies at 
about the same distance from North America as the 
Galapagos Islands do from South America, and which 
has a very peculiar soil, does not possess one endemic 
land bird; and we know from Mr. J. M. Jones's ad- 
mirable account of Bermuda, that very many North 
American birds, during their great annual migrations, 
visit either periodically or occasionally this island. 
Madeira does not possess one peculiar bird, and many 
European and African birds are almost every year blown 
there, as I am informed by Mr. E. V. Harcourt. So that 
these two islands of Bermuda and Madeira have been 
stocked by birds, which for long ages have struggled 
together in their former homes, and have become mutu- 
ally adapted to each other ; and when settled in their 
new homes, each kind will have been kept by the others 
to their proper places and habits, and will consequently 
have been little liable to modification. Madeira, again, 
is inhabited by a wonderful number of peculiar land- 
shells, whereas not one species of sea-shell is confined to 
its shores : now, though we do not know how sea-shells 
are dispersed, yet we can see that their eggs or larvae, 
perhaps attached to seaweed or floating timber, or to 
the feet of wading-birds, might be transported far more 
easily than land-shells, across three or four hundred 
miles of open sea. The different orders of insects in 
Madeira apparently present analogous facts. 

Oceanic islands are sometimes deficient in certain 
classes, and their places are apparently occupied by 
the other inhabitants ; in the Galapagos Islands reptiles, 
and in New Zealand gigantic wingless birds, take the 
place of mammals. In the plants of the Galapagos 
Islands, Dr. Hooker has shown that the proportional 
numbers of the different orders are very different from 


what they are elsewhere. Such cases are generally ac- 
counted for by the physical conditions of the islands ; 
but tins explanation seems to me not a little doubtful. 
Facility of immigration, I believe, has been at least as 
important as the nature of the conditions. 

Many remarkable little facts could be given with 
respect to the inhabitants of remote islands. For 
instance, in certain islands not tenanted by mammals, 
some of the endemic plants have beautifully hooked 
seeds; yet few relations are more striking than the 
adaptation of hooked seeds for transportal by the wool 
and fur of quadrupeds. This case presents no difficulty 
on my view, for a hooked seed might be transported to 
an island by some other means ; and the plant then 
becoming slightly modified, but still retaining its hooked 
seeds, would form an endemic species, having as useless 
an appendage as any rudimentary organ, — for instance, 
as the shrivelled wings under the soldered elytra of 
many insular beetles. Again, islands often possess trees 
or bushes belonging to orders which, elsewhere include 
only herbaceous species ; now trees, as Alph. de Can- 
dolle has shown, generally have, whatever the cause 
may be, confined ranges. Hence trees would be little 
likely to reach distant oceanic islands; and an herb- 
aceous plant, though it would have no chance of 
successfully competing in stature with a fully deve- 
loped tree, when established on an island and having 
to compete with herbaceous plants alone, might readily 
gain an advantage by growing taller and taller and 
overtopping the other plants. If so, natural selection 
would often tend to add to the stature of herbaceous 
plants when growing on an island, to whatever order 
they belonged, and thus convert them first into bushes 
and ultimately into trees. 

With respect to the absence of whole orders on 


oceanic islands, Bory St. Vincent long ago remarked 
that Batrachians (frogs, toads, newts) have never been 
found on any of the many islands with which the great 
oceans are studded. I have taken pains to verify this 
assertion, and I have found it strictly true. I have, 
however, been assured that a frog exists on the moun- 
tains of the great island of New Zealand ; but I suspect 
that this exception (if the information be correct) may 
be explained through glacial agency. This general 
absence of frogs, toads, and newts on so many oceanic 
islands cannot be accounted for by their physical con- 
ditions ; indeed it seems that islands are peculiarly well 
fitted for these animals ; for frogs have been introduced 
into Madeira, the Azores, and Mauritius, and have 
multiplied so as to become a nuisance. But as these 
animals and their spawn are known to be immediately 
killed by sea-water, on my view we can see that 
there would be great difficulty in their transportal 
across the sea, and therefore why they do not exist on 
any oceanic island. But why, on the theory of creation, 
they should not have been created there, it would be 
very difficult to explain. 

Mammals offer another and similar case. I have 
carefully searched the oldest voyages, but have not 
finished my search ; as yet I have not found a single 
instance, free from doubt, of a terrestrial mammal 
(excluding domesticated animals kept by the natives) 
inhabiting an island situated above 300 miles from a 
continent or great continental island ; and many islands 
situated at a much less distance are equally barren. 
The Falkland Islands, which are inhabited by a wolf- 
like fox, come nearest to an exception ; but this group 
cannot be considered as oceanic, as it lies on a bank con- 
nected with the mainland ; moreover, icebergs formerly 
brought boulders to its western shores, and they may 

s 3 


have formerly transported foxes, as so frequently now 
happens in the arctic regions. Yet it cannot be said 
that small islands will not support small mammals, for 
they occur in many parts of the world on very 
small islands, if close to a continent; and hardly an 
island can be named on which our smaller quadrupeds 
have not become naturalised and greatly multiplied. 
[It cannot be said, on the ordinary view of creation, 
that there has not been time for the creation of mam- 
mals ; many volcanic islands are sufficiently ancient, 
as shown by the stupendous degradation winch they 
have suffered and by their tertiary strata : there has 
also been time for the production of endemic species 
belonging to other classes ; and on continents it is 
thought that mammals appear and disappear at a 
quicker rate than other and lower animals. Though 
terrestrial mammals do not occur on oceanic islands, 
aerial mammals do occur on almost every island. New 
Zealand possesses two bats found nowhere else in the 
world : Norfolk Island, the Viti Archipelago, the Bonin 
Islands, the Caroline and Marianne Archipelagoes, and 
Mauritius, all possess their peculiar bats. Why, it may 
be asked, has the supposed creative force produced 
bats and no other mammals on remote islands? On 
my view tins question can easily be answered ; for no 
terrestrial mammal can be transported across a wide 
space of sea, but bats can fly across. Bats have been 
seen wandering by day far over the Atlantic Ocean; 
and two North American species either regularly or 
occasionally visit Bermuda, at the distance of 600 miles 
from the mainland. I hear from Mr. Tomes, who has 
specially studied this family, that many of the same 
species have enormous ranges, and are found on conti- 
nents and on far distant islands. Hence we have only 
to suppose that such wandering species have been modi- 


fied through natural selection in their new homes in 
relation to their new position, and we can understand 
the presence of endemic bats on islands, with the ab- 
sence of all terrestrial mammals. 

Besides the absence of terrestrial mammals in rela- 
tion to the remoteness of islands from continents, there 
is also a relation, to a certain extent independent of 
distance, between the depth of the sea separating an 
island from the neighbouring mainland, and the pre- 
sence in both of the same mammiferous species or of 
allied species in a more or less modified condition. Mr. 
Windsor Earl has made some striking observations on 
this head in regard to the great Malay Archipelago, 
which is traversed near Celebes by a space of deep 
ocean ; and this space separates two widely distinct 
mammalian faunas. On either side the islands are 
situated on moderately deep submarine banks, and they 
are inhabited by closely allied or identical quadrupeds. 
No doubt some few anomalies occur in this great archi- 
pelago, and there is much difficulty in forming a judg- 
ment in some cases owing to the probable naturalisation 
of certain mammals through man's agency ; but we 
shall soon have much light thrown on the natural 
history of this archipelago by the admirable zeal and 
researches of Mr. Wallace. I have not as yet had time to 
follow up this subject in all other quarters of the world ; 
but as far as I have gone, the relation generally holds 
good. We see Britain separated by a shallow channel 
from Europe, and the mammals are the same on both 
sides ; we meet with analogous facts on many islands 
separated by similar channels from Australia. The 
West Indian Islands stand on a deeply submerged bank, 
nearly 1000 fathoms in depth, and here we find American 
forms, but the species and even the genera are distinct. 
As the amount of modification in all cases depends to 


a certain degree on the lapse of time, and as during 
changes of level it is obvious that islands separated by 
shallow channels are more likely to have been con- 
tinuously united within a recent period to the main- 
land than islands separated by deeper channels, we can 
understand the frequent relation between the depth of 
the sea and the degree of affinity of the mammalian 
inhabitants of islands with those of a neighbouring con- 
tinent, — an inexplicable relation on the view of inde- 
pendent acts of creation. 

All the foregoing remarks on the inhabitants of 
oceanic islands, — namely, the scarcity of kinds — the 
richness in endemic forms in particular classes or sec- 
tions of classes, — the absence of whole groups, as of 
batrachians, and of terrestrial mammals notwithstanding 
the presence of aerial bats, — the singular proportions of 
certain orders of plants, — herbaceous forms having been 
developed into trees, &c, — seem to me to accord better 
with the view of occasional means of transport having 
been largely efficient in the long course of time, than 
with the view of all our oceanic islands having been 
formerly connected by continuous land with the nearest 
continent ; for on this latter view the migration would 
probably have been more complete ; and if modifi- 
cation be admitted, all the forms of life would have 
been more equally modified, in accordance with the 
paramount importance of the relation of organism to 

I do not deny that there are many and grave diffi- 
culties in understanding how several of the inhabitants 
of the more remote islands, whether still retaining the 
same specific form or modified since their arrival, could 
have reached their present homes. But the probability 
of many islands having existed as halting-places, of 
which not a wreck now remains, must not be over- 


looked. I will here give a single instance of one of 
the cases of difficulty. Almost all oceanic islands, 
even the most isolated and smallest, are inhabited by 
land-shells, generally by endemic species, but sometimes 
by species found elsewhere. Dr. Aug. A. Gould has 
given several interesting cases in regard to the land- 
shells of the islands of the Pacific. Now it is notorious 
that land-shells are very easily killed by salt; their 
eggs, at least such as I have tried, sink in sea-water 
and are killed by it. Yet there must be, on my view, 
some unknown, but highly efficient means for their trans- 
portal. Would the just-hatched young occasionally 
crawl on and adhere to the feet of birds roosting on the 
ground, and thus get transported ? It occurred to 
me that land-shells, when hybernating and having a 
membranous diaphragm over the mouth of the shell, 
might be floated in chinks of drifted timber across 
moderately wide arms of the sea. And I found that 
several species did in this state withstand uninjured an 
immersion in sea-water during seven days : one of these 
shells was the Helix pomatia, and after it had again 
hybernated I put it in sea-water for twenty days, 
and it perfectly recovered. As this species has a 
thick calcareous operculum, I removed it, and when it 
had formed a new membranous one, I immersed it for 
fourteen days in sea-water, and it recovered and 
crawled away : but more experiments are wanted on 
this head. 

The most striking and important fact for us in regard 
to the inhabitants of islands, is their affinity to those of 
the nearest mainland, without being actually the same 
species. Numerous instances could be given of this 
fact. I will give only one, that of the Galapagos 
Archipelago, situated under the equator, between 500 
and 600 miles from the shores of South America. Here 


almost every product of the land and water bears the 
unmistakeable stamp of the American continent. There 
are twenty-six land birds, and twenty-five of these are 
ranked by Mr. Gould as distinct species, supposed to 
have been created here ; yet the close affinity of most 
of these birds to American species in every character, 
in their habits, gestures, and tones of voice, was mani- 
fest. So it is with the other animals, and with nearly 
all the plants, as shown by Dr. Hooker in his admirable 
memoir on the Flora of this archipelago. The natu- 
ralist, looking at the inhabitants of these volcanic 
islands in the Pacific, distant several hundred miles 
from the continent, yet feels that he is standing on 
American land. Why should this be so? why should 
the species which are supposed to have been created in 
the Galapagos Archipelago, and nowhere else, bear so 
plain a stamp of affinity to those created in America ? 
There is notlring in the conditions of life, in the geo- 
logical nature of the islands, in their height or climate, 
or in the proportions in winch the several classes are 
associated together, which resembles closely the con- 
ditions of the South American coast : in fact there is 
a considerable dissimilarity in all these respects. On 
the other hand, there is a considerable degree of re- 
semblance in the volcanic nature of the soil, in climate, 
height, and size of the islands, between the Galapagos 
and Cape de Verde Archipelagos : but what an entire 
and absolute difference in their inhabitants! The in- 
habitants of the Cape de Verde Islands are related to 
those of Africa, like those of the Galapagos to America. 
I believe this grand fact can receive no sort of expla- 
nation on the ordinary view of independent creation ; 
whereas on the view here maintained, it is obvious 
that the Galapagos Islands would be likely to receive 
colonists, whether by occasional means of transport or 


by formerly continuous land, from America ; and the 
Cape de Yerde Islands from Africa; and that such 
colonists would be liable to modification ; — the principle 
of inheritance still betraying their original birthplace. 

Many analogous facts could be given : indeed it is an 
almost universal rule that the endemic productions of 
islands are related to those of the nearest continent, or 
of other near islands. The exceptions are few, and 
most of them can be explained. Thus the plants of 
Kerguelen Land, though standing nearer to Africa than 
to America, are related, and that very closely, as we 
know from Dr. Hooker's account, to those of America : 
but on the view that this island has been mainly stocked 
by seeds brought with earth and stones on icebergs, 
drifted by the prevailing currents, this anomaly dis- 
appears. New Zealand in its endemic plants is much 
more closely related to Australia, the nearest mainland, 
than to any other region : and this is what might have 
been expected ; but it is also plainly related to South 
America, which, although the next nearest continent, 
is so enormously remote, that the fact becomes an 
anomaly. But this difficulty almost disappears on the 
view that both New Zealand, South America, and 
other southern lands were long ago partially stocked 
from a nearly intermediate though distant point, namely 
from the antarctic islands, when they were clothed 
with vegetation, before the commencement of the Gla- 
cial period. The affinity, which, though feeble, I am 
assured by Dr. Hooker is real, between the flora of 
the south-western corner of Australia and of the Cape 
of Good Hope, is a far more remarkable case, and is 
at present inexplicable : but this affinity is confined to 
the plants, and will, I do not doubt, be some day ex- 

The law which causes the inhabitants of an archi- 


pelago, though specifically distinct, to be closely allied 
to those of the nearest continent, we sometimes see 
displayed on a small scale, yet in a most interesting 
manner, within the limits of the same archipelago. 
Thus the several islands of the Galapagos Archipelago 
are tenanted, as I have elsewhere shown, in a quite 
marvellous manner, by very closely related species ; 
so that the inhabitants of each separate island, though 
mostly distinct, are related in an incomparably closer 
degree to each other than to the inhabitants of any 
other part of the world. And this is just what might 
have been expected on my view, for the islands are 
situated so near each other that they would almost 
certainly receive immigrants from the same original 
source, or from each other. But this dissimilarity 
between the endemic inhabitants of the islands may 
be used as an argument against my views ; for it may 
be asked, how has it happened in the several islands 
situated within sight of each other, having the same 
geological nature, the same height, climate, &c, that 
many of the immigrants should have been differently 
modified, though only in a small degree. This long 
appeared to me a great difficulty : but it arises in 
chief part from the deeply-seated error of considering 
the physical conditions of a country as the most im- 
portant for its inhabitants ; whereas it cannot, I think, 
be disputed that the nature of the other inhabitants, 
with which each has to compete, is at least as impor- 
tant, and generally a far more important element of 
success. Now if we look to those inhabitants of the 
Galapagos Archipelago which are found in other parts 
of the world (laying on one side for the moment the 
endemic species, which cannot be here fairly included, 
as we are considering how they have come to be modi- 
fied since their arrival), we find a considerable amount 


of difference in the several islands. This difference 
might indeed have been expected on the view of the 
islands having been stocked by occasional means of 
transport — a seed, for instance, of one plant having 
been brought to one island, and that of another plant 
to another island. Hence when in former times an 
immigrant settled on any one or more of the islands, or 
when it subsequently spread from one island to another, 
it would undoubtedly be exposed to different conditions 
of life in the different islands, for it would have to 
compete with different sets of organisms : a plant, for 
instance, would find the best-fitted ground more per- 
fectly occupied by distinct plants in one island than 
in another, and it would be exposed to the attacks of 
somewhat different enemies. If then it varied, natural 
selection would probably favour different varieties in 
the different islands. Some species, however, might 
spread and yet retain the same character throughout 
the group, just as we see on continents some species 
spreading widely and remaining the same. 

The really surprising fact in this case of the Gala- 
pagos Archipelago, and in a lesser degree in some 
analogous instances, is that the new species formed in 
the separate islands have not quickly spread to the other 
islands. But the islands, though in sight of each other, 
are separated by deep arms of the sea, in most cases 
wider than the British Channel, and there is no reason 
to suppose that they have at any former period been 
continuously united. The currents of the sea are rapid 
and sweep across the archipelago, and gales of wind 
are extraordinarily rare ; so that the islands are far 
more effectually separated from each other than they 
appear to be on a map. Nevertheless a good many 
species, both those found in other parts of the world 
and those confined to the archipelago, are common to 


the several islands, and we may infer from certain facts 
that these have probably spread from some one island 
to the others. But we often take, I think, an erro- 
neous view of the probability of closely allied species 
invading each other's territory, when put into free 
intercommunication. Undoubtedly if one species has 
any advantage whatever over another, it will in a 
very brief time wholly or in part supplant it; but if 
both are equally well fitted for their own places in 
nature, both probably will hold their own places and 
keep separate for almost any length of time. Being 
familiar with the fact that many species, naturalised 
through man's agency, have spread with astonishing 
rapidity over new countries, we are apt to infer that 
most species would thus spread ; but we should remem- 
ber that the forms which become naturalised in new 
countries are not generally closely allied to the aboriginal 
inhabitants, but are very distinct species, belonging in a 
large proportion of cases, as shown by Alph. de Candolle, 
to distinct genera. In the Galapagos Archipelago, many 
even of the birds, though so well adapted for flying 
from island to island, are distinct on each ; thus there 
are three closely-allied species of mocking-thrush, each 
confined to its own island. Now let us suppose the 
mocking-thrush of Chatham Island to be blown to 
Charles Island, which has its own mocking-thrush : why 
should it succeed in establishing itself there ? We may 
6afely infer that Charles Island is well stocked with its 
own species, for annually more eggs are laid there 
than can possibly be reared ; and we may infer that the 
mocking-thrush peculiar to Charles Island is at least as 
well fitted for its home as is the species peculiar to 
Chatham Island. Sir C. Lyell and Mr. Wollaston have 
communicated to me a remarkable fact bearing on this 
subject ; namely, that Madeira and the adjoining islet of 


Porto Santo possess many distinct but representative 
land-shells, some of which live in crevices of stone ; and 
although large quantities of stone are annually trans- 
ported from Porto Santo to Madeira, yet this latter 
island has not become colonised by the Porto Santo 
species : nevertheless both islands have been colonised 
by some European land-shells, which no doubt had some 
advantage over the indigenous species. From these 
considerations I think we need not greatly marvel at 
the endemic and representative species, which inhabit 
the several islands of the Galapagos Archipelago, not 
having universally spread from island to island. In 
many other instances, as in the several districts of the 
same continent, pre-occupation has probably played an 
important part in checking the commingling of species 
under the same conditions of life. Thus, the south-east 
and south-west corners of Australia have nearly the 
same physical conditions, and are united by continuous 
land, yet they are inhabited by a vast number of distinct 
mammals, birds, and plants. 

The principle which determines the general character 
of the fauna and flora of oceanic islands, namely, that 
the inhabitants, when not identically the same, yet are 
plainly related to the inhabitants of that region whence 
colonists could most readily have been derived, — the 
colonists having been subsequently modified and better 
fitted to their new homes, — is of the widest applica- 
tion throughout nature. We see this on every moun- 
tain, in every lake and marsh. For Alpine species, 
excepting in so far as the same forms, chiefly of plants, 
have spread widely throughout the world during the 
recent Glacial epoch, are related to those of the sur- 
rounding lowlands ; — thus we have in South America, 
Alpine humming-birds, Alpine rodents, Alpine plants, 
&c, all of strictly American forms, and it is obvious 


that a mountain, as it became slowly upheaved, would 
naturally be colonised from the surrounding lowlands. 
So it is with the inhabitants of lakes and marshes, 
excepting in so far as great facility of transport has 
given the same general forms to the whole world. We 
see this same principle in the blind animals inhabiting 
the caves of America and of Europe. Other analogous 
facts could be given. And it will, I believe, be uni- 
versally found to be true, that wherever in two regions, 
let them be ever so distant, many closely allied or re- 
presentative species occur, there will likewise be found 
some identical species, showing, in accordance with the 
foregoing view, that at some former period there has 
been intercommunication or migration between the two 
regions. And wherever many closely-allied species 
occur, there will be found many forms winch some 
naturalists rank as distinct species, and some as varie- 
ties ; these doubtful forms showing us the steps in the 
process of modification. 

This relation between the power and extent of migra- 
tion of a species, either at the present time or at some 
former period under different physical conditions, and 
the existence at remote points of the world of other 
species allied to it, is shown in another and more 
general way. Mr. Gould remarked to me long ago, that 
in those genera of birds which range over the world, 
many of the species have very wide ranges. I can 
hardly doubt that this rule is generally true, though it 
would be difficult to prove it. Amongst mammals, we 
see it strikingly displayed in Bats, and in a lesser degree 
in the Felidae and Canidae. We see it, if we compare 
the distribution of butterflies and beetles. So it is with 
most fresh- water productions, in which so many genera 
range over the world, and many individual species have 
enormous ranges. It is not meant that in world- 


ranging genera all the species have a wide range, or 
even that they have on an average a wide range ; but 
only that some of the species range very widely ; for the 
facility with which widely-ranging species vary and give 
rise to new forms will largely determine their average 
range. For instance, two varieties of the same species 
inhabit America and Europe, and the species thus has 
an immense range ; but, if the variation had been a little 
greater, the two varieties would have been ranked as dis- 
tinct species, and the common range would have been 
greatly reduced. Still less is it meant, that a species 
which apparently has the capacity of crossing barriers 
and ranging widely, as in the case of certain powerfully- 
winged birds, will necessarily range widely ; for we 
should never forget that to range widely implies not 
only the power of crossing barriers, but the more im- 
portant power of being victorious in distant lands in 
the struggle for life with foreign associates. But 
on the view of all the species of a genus having de- 
descended from a single parent, though now distributed 
to the most remote points of the world, we ought to 
find, and I believe as a general rule we do find, that 
some at least of the species range very widely ; for it is 
necessary that the unmodified parent should range 
widely, undergoing modification during its diffusion, and 
should place itself under diverse conditions favourable 
for the conversion of its offspring, firstly into new varie- 
ties and ultimately into new species. 

In considering the wide distribution of certain genera, 
we should bear in mind that some are extremely ancient, 
and must have branched off from a common parent 
at a remote epoch ; so that in such cases there will 
have been ample time for great climatal and geographical 
changes and for accidents of transport; and conse- 
quently for the migration of some of the species into all 


quarters of the world, where they may have become 
slightly modified in relation to their new conditions. 
There is, also, some reason to believe from geological 
evidence that organisms low in the scale within each 
great class, generally change at a slower rate than the 
higher forms ; and consequently the lower forms will have 
had a better chance of ranging widely and of still re- 
taining the same specific character. This fact, together 
with the seeds and eggs of many low forms being very 
minute and better fitted for distant transportation, pro- 
bably accounts for a law which has long been observed, 
and which has lately been admirably discussed by Alph. de 
Candolle in regard to plants, namely, that the lower any 
group of organisms is, the more widely it is apt to range. 
The relations just discussed, — namely, low and slowly- 
changing organisms ranging more widely than the 
high, — some of the species of widely-ranging genera 
themselves ranging widely, — such facts, as alpine, lacus- 
trine, and marsh productions being related (with the 
exceptions before specified) to those on the surrounding 
low lands and dry lands, though these stations are so 
different — the very close relation of the distinct species 
which inhabit the islets of the same archipelago, — and 
especially the striking relation of the inhabitants of 
each whole archipelago or island to those of the nearest 
mainland, — are, I think, utterly inexplicable on the 
ordinary view of the independent creation of each spe- 
cies, but are explicable on the view of colonisation 
from the nearest and readiest source, together with the 
subsequent modification and better adaptation of the 
colonists to their new homes. 

Summary of last and present Chapters. — In these 
chapters I have endeavoured to show, that if wo make 
due allowance for our ignorance of the full effects of all 

Chap. XII. SUMMARY. 407 

the changes of climate and of the level of the land, 
which have certainly occurred within the recent period, 
and of other similar changes which may have occurred 
within the same period ; if we remember how pro- 
foundly ignorant we are with respect to the many 
and curious means of occasional transport, — a subject 
which has hardly ever been properly experimentised 
on ; if we bear in mind how often a species may have 
ranged continuously over a wide area, and then have 
become extinct in the intermediate tracts, I think the 
difficulties in believing that all the individuals of the 
same species, wherever located, have descended from the 
same parents, are not insuperable. And we are led to 
this conclusion, which has been arrived at by many 
naturalists under the designation of single centres of 
creation, by some general considerations, more especially 
from the importance of barriers and from the analogical 
distribution of sub-genera, genera, and families. 

With respect to the distinct species of the same genus, 
which on my theory must have spread from one parent- 
somce ; if we make the same allowances as before for 
our ignorance, and remember that some forms of life 
change most slowly, enormous periods of time being 
thus granted for their migration, I do not think that the 
difficulties are insuperable ; though they often are in 
this case, and in that of the individuals of the same spe- 
cies, extremely grave. 

As exemplifying the effects of climatal changes on 
distribution, I have attempted to show how important 
has been the influence of the modern Glacial period, 
which I am fully convinced simultaneously affected the 
whole world, or at least great meridional belts. As 
showing how diversified are the means of occasional 
transport, I have discussed at some little length the 
means of dispersal of fresh-water productions. 


If the difficulties be not insuperable in admitting 
that in the long course of time the individuals of the 
same species, and likewise of allied species, have pro- 
ceeded from some one source ; then I think all the grand 
leading facts of geographical distribution are explicable 
on the theory of migration (generally of the more do- 
minant forms of life), together with subsequent modifi- 
cation and the multiplication of new forms. We can 
thus understand the high importance of barriers, whether 
of land or water, which separate our several zoological 
and botanical provinces. We can thus understand the 
localisation of sub-genera, genera, and families ; and how 
it is that under different latitudes, for instance in South 
America, the inhabitants of the plains and mountains, of 
the forests, marshes, and deserts, are in so mysterious 
a manner linked together by affinity, and are likewise 
linked to the extinct beings which formerly inhabited the 
same continent. Bearing in mind that the mutual rela- 
tions of organism to organism are of the highest import- 
ance, we can see why two areas having nearly the same 
physical conditions should often be inhabited by very 
different forms of life ; for according to the length of time 
which has elapsed since new inhabitants entered one 
region ; according to the nature of the communication 
which allowed certain forms and not others to enter, either 
in greater or lesser numbers ; according or not, as those 
which entered happened to come in more or less 'direct 
competition with each other and with the aborigines ; 
and according as the immigrants were capable of vary- 
ing more or less rapidly, there would ensue in different 
regions, independently of their physical conditions, infi- 
nitely diversified conditions of life, — there would be an 
almost endless amount of organic action and reaction, — 
and we should find, as we do find, some groups of beings 
greatly, and some only slightly modified, — some deve- 

Chap. XII. SUMMARY. 409 

loped in great force, some existing in scanty numbers — 
in the different great geographical provinces of the 

On these same principles, we can understand, as I 
have endeavoured to show, why oceanic islands should 
have few inhabitants, but of these a great number 
should be endemic or peculiar ; and why, in relation to 
the means of migration, one group of beings, even within 
the same class, should have all its species endemic, and 
another group should have all its species common to 
other quarters of the world. We can see why whole 
groups of organisms, as batrachians and terrestrial mam- 
mals, should be absent from oceanic islands, whilst the 
most isolated islands possess their own peculiar species of 
aerial mammals or bats. We can see why there should 
be some relation between the presence of mammals, in 
a more or less modified condition, and the depth of 
the sea between an island and the mainland. We can 
clearly see why all the inhabitants of an archipelago, 
though specifically distinct on the several islets, should 
be closely related to each other, and likewise be related, 
but less closely, to those of the nearest continent or 
other source whence immigrants were probably derived. 
We can see why in two areas, however distant from each 
other, there should be a correlation, in the presence of 
identical species, of varieties, of doubtful species, and of 
distinct but representative species. 

As the late Edward Forbes often insisted, there is a 
striking parallelism in the laws of life throughout time 
and space : the laws governing the succession of forms in 
past times being nearly the same with those governing 
at the present time the differences in different areas. 
We see this in many facts. The endurance of each 
species and group of species is continuous in time ; for 
the exceptions to the rule are so few, that they may 



fairly be attributed to our not having as yet discovered 
in an intermediate deposit the forms which are therein 
absent, but which occur above and below : so in space, 
it certainly is the general rule that the area inha- 
bited by a single species, or by a group of species, is 
continuous ; and the exceptions, which are not rare, 
may, as I have attempted to show, be accounted for by 
migration at some former period under different con- 
ditions or by occasional means of transport, and by the 
species having become extinct in the intermediate tracts. 
Both in time and space, species and groups of species 
have their points of maximum development. Groups of 
species, belonging either to a certain period of time, or 
to a certain area, are often characterised by trifling cha- 
racters in common, as of sculpture or colour. In look- 
ing to the long succession of ages, as in now looking to 
distant provinces throughout the world, we find that 
some organisms differ little, whilst others belonging to a 
different class, or to a different order, or even only to a 
different family of the same order, differ greatly. In 
both time and space the lower members of each class 
generally change less than the higher; but there are 
in both cases marked exceptions to the rule. On my 
theory these several relations throughout time and space 
are intelligible ; for whether we look to the forms of life 
which have changed during successive ages within the 
same quarter of the world, or to those which have 
changed after having migrated into distant quarters, in 
both cases the forms within each class have been con- 
nected by the same bond of ordinary generation ; and 
the more nearly any two forms are related in blood, the 
nearer they will generally stand to each other in time 
and space ; in both cases the laws of variation have been 
the same, and modifications have been accumulated by 
the same power of natural selection. 



Mutual Affinities of Organic Beings : Morphology : 
Embryology : Rudimentary Organs. 

Classification, groups subordinate to groups — Natural system — 
Rules and difficulties in classification, explained on the theory of 
descent with modification — Classification of varieties — Descent 
always used in classification — •Analogical or adaptive characters 
— Affinities, general, complex and radiating — Extinction 
separates and defines groups — Morphology, between members 
of the same class, between parts of the same individual — 
Embryology, laws of, explained by variations not supervening 
at an early age, and being inherited at a corresponding age — 
Rudimentary organs ; their origin explained — Summary. 

From the first dawn of life, all organic beings are found 
to resemble each other in descending degrees, so that 
they can be classed in groups under groups. This classi- 
fication is evidently not arbitrary like the grouping of 
the stars in constellations. The existence of groups 
would have been of simple signification, if one group had 
been exclusively fitted to inhabit the land, and another 
the water ; one to feed on flesh, another on vegetable 
matter, and so on ; but the case is widely different in 
nature; for it is notorious how commonly members of 
even the same sub-group have different habits. In 
our second and fourth chapters, on Variation and on 
Natural Selection, I have attempted to show that it is 
the widely rangiug, the much diffused and common, that 
is the dominant species belonging to the larger genera, 
which vary most. The varieties, or incipient species, 
thus produced ultimately become converted, as I believe, 
into new and distinct species ; and these, on the principle 
of inheritance, tend to produce other new and dominant 

T 2 


species. Consequently the groups which are now large, 
and which generally include many dominant species, 
tend to go on increasing indefinitely in size. I further 
attempted to show that from the varying descendants of 
each species trying to occupy as many and as different 
places as possible in the economy of nature, there is a 
constant tendency in their characters to diverge. This 
conclusion was supported by looking at the great diversity 
of the forms of life which, in any small area, come into 
the closest competition, and by looking to certain facts 
in naturalisation. 

I attempted also to show that there is a constant 
tendency in the forms which are increasing in number 
and diverging in character, to supplant and exterminate 
the less divergent, the less improved, and preceding 
forms. I request the reader to turn to the diagram 
illustrating the action, as formerly explained, of these 
several principles; and he will see that the inevitable 
result is that the modified descendants proceeding from 
one progenitor become broken up into groups subordi- 
nate to groups. In the diagram each letter on the 
uppermost line may represent a genus including several 
species ; and all the genera on this line form together 
one class, for all have descended from one ancient 
but unseen parent, and, consequently, have inherited 
something in common. But the three genera on the 
left hand have, on this same principle, much in common, 
and form a sub-family, distinct from that including the 
next two genera on the right hand, which diverged from 
a common parent at the fifth stage of descent. These 
five genera have also much, though less, in common ; 
and they form a family distinct from that including 
the three genera still further to the right hand, which 
diverged at a still earlier period. And all these genera, 
descended from (A), form an order distinct from the 


genera descended from (I). So that we here have many 
species descended from a single progenitor grouped into 
genera ; and the genera are included in, or subordinate to, 
sub-families, families, and orders, all united into one class. 
Thus, the grand fact in natural history of the subordi- 
nation of group under group, which, from its familiarity, 
does not always sufficiently strike us, is in my judgment 
fully explained. 

Naturalists try to arrange the species, genera, and 
families in each class, on what is called the Natural 
System. But what is meant by this system ? Some 
authors look at it merely as a scheme for arranging to- 
gether those living objects which are most alike, and for 
separating those which are most unlike ; or as an artificial 
means for enunciating, as briefly as possible, general pro- 
positions, — that is, by one sentence to give the charac- 
ters common, for instance, to all mammals, by another 
those common to all carnivora, by another those com- 
mon to the dog-genus, and then by adding a single sen- - 
tence, a full description is given of each kind of dog. 
The ingenuity and utility of this system are indisputable. 
But many naturalists think that something more is meant 
by the Natural System ; they believe that it reveals the 
plan of the Creator ; but unless it be specified whether 
order in time or space, or what else is meant by the plan 
of the Creator, it seems to me that nothing is thus added 
to our knowledge. Such expressions as that famous one 
of Linnaeus, and which we often meet with in a more or 
less concealed form, that the characters do not make the 
genus, but that the genus gives the characters, seem to 
imply that something more is included in our classifica- 
tion, than mere resemblance. I believe that something 
more is included ; and that propinquity of descent, — the 
only known cause of the similarity of organic beings, — 
is the bond, hidden as it is by various degrees of niodifi- 


cation, which is partially revealed to us by our classifi- 

Let us now consider the rules followed in classi- 
fication, and the difficulties which are encountered on 
the view that classification either gives some unknown 
plan of creation, or is simply a scheme for enunciating 
. general propositions and of placing together the forms 
most like each other. It might have been thought (and 
was in ancient times thought) that those parts of the 
structure which determined the habits of life, and the 
general place of each being in the economy of nature, 
would be of very high importance in classification. 
Nothing can be more false. No one regards the external 
similarity of a mouse to a shrew, of a dugong to a whale, 
of a whale to a fish, as of any importance. These resem- 
blances, though so intimately connected with the whole 
life of the being, are ranked as merely "adaptive or 
analogical characters ; " but to the consideration of these 
resemblances we shall have to recur. It may even be 
given as a general rule, that the less any part of the 
organisation is concerned with special habits, the more 
important it becomes for classification. As an instance : 
Owen, in speaking of the dugong, says, " The generative 
organs being those which are most remotely related to the 
habits and food of an animal, I have always regarded as 
affording very clear indications of its true affinities. We 
are least likely in the modifications of these organs to 
mistake a merely adaptive for an essential character." 
/ So with plants, how remarkable it is that the organs of 
vegetation, on which their whole life depends, are of 
little signification, excepting in the first main divisions ; 
whereas the organs of reproduction, with their product 
the seed, are of paramount importance ! 

We must not, therefore, in classifying, trust to resem- 
blances in parts of the organisation, however important 


they may be for the welfare of the being in relation to 
the outer world. Perhaps from this cause it has partly 
arisen, that almost all naturalists lay the greatest stress 
on resemblances in organs of high vital or physiological 
importance. No doubt this view of the classificatory im- 
portance of organs which are important is generally, but 
by no means always, true. But their importance for 
classification, I believe, depends on their greater con- 
stancy throughout large groups of species ; and this con- 
stancy depends on such organs having generally been 
subjected to less change in the adaptation of the species 
to their conditions of life. That the mere physiological 
importance of an organ does not determine its classi- 
ficatory value, is almost shown by the one fact, that in 
allied groups, in which the same organ, as we have every 
reason to suppose, has nearly the same physiological 
value, its classificatory value is widely different. No 
naturalist can have worked at any group without being 
struck with this fact; and it has been most fully ac- 
knowledged in the writings of almost every author. It 
will suffice to quote the highest authority, Kobert 
Brown, who in speaking of certain organs in the Pro- 
teacese, says their generic importance, " like that of all 
their parts, not only in this but, as I apprehend, in 
every natural family, is very unequal, and in some cases 
seems to be entirely lost." Again in another work he 
says, the genera of the Connaraceoe " differ in having 
one or more ovaria, in the existence or absence of al- 
bumen, in the imbricate or valvular estivation. Any 
one of these characters singly is frequently of more than 
generic importance, though here even when all taken 
together they appear insufficient to separate Cnestis from 
Connarus." To give an example amongst insects, in 
one great division of the Hymenoptera, the antennae, as 
Westwood has remarked, are most constant in structure ; 


in another division they differ much, and the differences 
are of quite subordinate value in classification ; yet no 
one probably will say that the antennae in these two 
divisions of the same order are of unequal physiological 
importance. Any number of instances could be given 
of the varying importance for classification of the same 
important organ within the same group of beings. 

Again, no one will say that rudimentary or atrophied 
organs are of high physiological or vital importance; 
yet, undoubtedly, organs in this condition are often of 
high value in classification. No one will dispute that 
the rudimentary teeth in the upper jaws of young rumi- 
nants, and certain rudimentary bones of the leg, are 
highly serviceable in exhibiting the close affinity be- 
tween Ruminants and Pachyderms. Robert Brown has 
strongly insisted on the fact that the rudimentary florets 
are of the highest importance in the classification of the 

Numerous instances could be given of characters 
derived from parts which must be considered of very 
trifling physiological importance, but which are univer- 
sally admitted as highly serviceable in the definition 
of whole groups. For instance, whether or not there is 
an open passage from the nostrils to the mouth, the 
only character, according to Owen, winch absolutely dis- 
tinguishes fishes and reptiles — the inflection of the angle 
of the jaws in Marsupials — the manner in which the 
wings of insects are folded — mere colour in certain 
Algge— mere pubescence on parts of the flower in 
grasses — the nature of the dermal covering, as hair or 
feathers, in the Yertebrata. If the Ornithorhynehus had 
been covered with feathers instead of hair, this external 
and trifling character would, I think, have been con- 
sidered by naturalists as important an aid in deter- 
mining the degree of affinity of this strange creature to 


birds and reptiles, as an approach in structure in any one 
internal and important organ. 

The importance, for classification, of trifling characters, 
mainly depends on their being correlated with several 
other characters of more or less importance. The value 
indeed of an aggregate of characters is very evident in 
natural history. Hence, as has often been remarked, a 
species may depart from its allies in several characters, 
both of high physiological importance and of almost 
universal prevalence, and yet leave us in no doubt where 
it should be ranked. Hence, also, it has been found, 
that a classification founded on any single character, 
however important that may be, has always failed ; for 
no part of the organisation is universally constant. The 
importance of an aggregate of characters, even when 
none are important, alone explains, I think, that saying 
of Linnaeus, that the characters do not give the genus, 
but the genus gives the characters ; for this saying 
seems founded on an appreciation of many trifling points 
of resemblance, too slight to be defined. Certain plants, 
belonging to the Malpighiacese, bear perfect and de- 
graded flowers; in the latter, as A. de Jussieu has 
remarked, " the greater number of the characters proper 
to the species, to the genus, to the family, to the class, 
disappear, and thus laugh at our classification." But 
when Aspicarpa produced in France, during several 
years, only degraded flowers, departing so wonderfully 
in a number of the most important points of structure 
from the proper type of the order, yet M. Eichard 
sagaciously saw, as Jussieu observes, that this genus 
should still be retained amongst the Malpighiacese. 
This case seems to me well to illustrate the spirit with 
which our classifications are sometimes necessarily 

Practically when naturalists are at work, they do 

T 3 


not trouble themselves about the physiological value 
of the characters which they use in defining a group, 
or in allocating any particular species. If they find 
a character nearly uniform, and common to a great 
number of forms, and not common to others, they use 
it as one of high value ; if common to some lesser 
number, they use it as of subordinate value. This 
principle has been broadly confessed by some naturalists 
to be the true one ; and by none more clearly than by 
that excellent botanist, Aug. St. Hilaire. If certain 
characters are always found correlated with others, 
though no apparent bond of connexion can be dis- 
covered between them, especial value is set on them. 
As in most groups of animals, important organs, such as 
those for propelling the blood, or for aerating it, or those 
for propagating the race, are found nearly uniform, they 
are considered as highly serviceable in classification ; 
but in some groups of animals all these, the most im- 
portant vital organs, are found to offer characters of quite 
subordinate value. 

We can see why characters derived from the embryo 
should be of equal importance with those derived from 
the adult, for our classifications of course include all 
ages of each species. But it is by no means obvious, 
on the ordinary view, why the structure of the embryo 
should be more important for this purpose than that of 
the adult, which alone plays its full part in the economy 
of nature. Yet it has been strongly urged by those 
great naturalists, Milne Edwards and Agassiz, that em- 
bryonic characters are the most important of any in the 
classification of animals ; and this doctrine has very 
generally been admitted as true. The same fact holds 
good with flowering plants, of which the two main divi- 
sions have been founded on characters derived from 
the embryo, — on the number and position of the em- 


bryonic leaves or cotyledons, and on the mode of deve- 
lopment of the plumule and radicle. In our discussion 
on embryology, we shall see why such characters are so 
valuable, on the view of classification tacitly including 
the idea of descent. 

Our classifications are often plainly influenced by 
chains of affinities. Nothing can be easier than to 
define a number of characters common to all birds ; but 
in the case of crustaceans, such definition has hitherto 
been found impossible. There are crustaceans at the 
opposite ends of the series, which have hardly a cha- 
racter in common ; yet the species at both ends, from 
being plainly allied to others, and these to others, and 
so onwards, can be recognised as unequivocally belonging 
to this, and to no other class of the Articulata. 

Geographical distribution has often been used, though 
perhaps notjguite logically, in classification, more especi- 
ally in very large groups of closely allied forms. Tem- 
minck insists on the utility or even necessity of this 
practice in certain groups of birds ; and it has been 
followed by several entomologists and botanists. 

Finally, with respect to the comparative value of the 
various groups of species, such as orders, sub-orders, 
families, sub-families, and genera, they seem to be, at 
least at present, almost arbitrary. Several of the best 
botanists, such as Mr. Bentham and others, have 
strongly insisted on their arbitrary value. Instances 
could be given amongst plants and insects, of a group 
of forms, first ranked by practised naturalists as only a 
genus, and then raised to the rank of a sub-family or 
family ; and this has been done, not because further 
research has detected important structural differences, 
at first ovei looked, but because numerous allied species, 
with slightly different grades of difference, have been 
subsequently discovered. 


All the foregoing rules and aids and difficulties in 
classification are explained, if I do not greatly deceive 
myself, on the view that the natural system is founded 
/ on descent with modification ; that the characters which 
naturalists consider as showing true affinity between 
any two or more species, are those which have been 
inherited from a common parent, and, in so far, all true 
classification is genealogical ; that community of descent 
is the hidden bond which naturalists have been un- 
consciously seeking, and not some unknown plan of 
/creation, or the enunciation of general propositions, and 
j the mere putting together and separating objects more 
or less alike. 

But I must explain my meaning more fully. I 
believe that the arrangement of the groups within each 
class, in due subordination and relation to the other 
groups, must be strictly genealogical in order to be 
natural ; but that the amount of difference in the several 
branches or groups, though allied in the same degree in 
blood to their common progenitor, may differ greatly, 
being due to the different degrees of modification 
which they have undergone ; and this is expressed 
by the forms being ranked under different genera, 
families, sections, or orders. The reader will best 
understand what is meant, if he will take the trouble 
of referring to the diagram in the fourth chapter. We 
will suppose the letters A to L to represent allied 
genera, winch lived during the Silurian epoch, and these 
have descended from a species which existed at an un- 
known anterior period. Species of three of these genera 
(A, F, and I) have transmitted modified descendants to 
the present day, represented by the fifteen genera (a 14 to 
g») on the uppermost horizontal line. Now all these 
modified descendants from a single species, are repre- 
sented as related in blood or descent to the same 


degree ; they may metaphorically be called cousins to 
the same millionth degree ; yet they differ widely 
and in different degrees from each other. The forms 
descended from A, now broken up into two or three 
families, constitute a distinct order from those de- 
scended from I, also broken up into two families. Nor 
can the existing species, descended from A, be ranked 
in the same genus with the parent A ; or those from 
I, with the parent I. But the existing genus F 14 may 
be supposed to have been but slightly modified ; and 
it will then rank with the parent-genus F ; just as 
some few still living organic beings belong to Silurian 
genera. So that the amount or value of the differ- 
ences between organic beings all related to each other 
in the same degree in blood, has come to be widely 
different. Nevertheless their genealogical arrange- 
ment remains strictly true, not only at the present 
time, but at each successive period of descent. All 
the modified descendants from A will have inherited 
something in common from their common parent, as 
will all the descendants from I ; so will it be with each 
subordinate branch of descendants, at each successive 
period. If, however, we choose to suppose that any of 
the descendants of A or of I have been so much modi- 
fied as to have more or less completely lost traces of 
then parentage, in this case, their places in a natural 
classification will have been more or less completely lost, 
— as sometimes seems to have occurred with existing 
organisms. All the descendants of the genus F, along 
its whole line of descent, are supposed to have been 
but little modified, and they yet form a single genus. 
But this genus, though much isolated, will still occupy 
its proper intermediate position ; for F originally was 
intermediate in character between A and I, and the 
several genera descended from these two genera will 


have inherited to a certain extent their characters. 
This natural arrangement is shown, as far as is possible 
on paper, in the diagram, but in much too simple a 
manner. If a branching diagram had not been used, 
and only the names of the groups had been written in 
a linear series, it would have been still less possible to 
have given a natural arrangement ; and it is notoriously 
not possible to represent in a series, on a flat surface, 
the affinities which we discover in nature amongst the 
beings of the same group. Thus, on the view which I 
hold, the natural system is genealogical in its arrange- 
ment, like a pedigree ; but the degrees of modification 
which the different groups have undergone, have to be 
expressed by ranking them under different so-called 
genera, sub - families, families, sections, orders, and 

It may be worth while to illustrate this view of classi- 
fication, by taking the case of languages. If we pos- 
sessed a perfect pedigree of mankind, a genealogical 
arrangement of the races of man would afford the best 
classification of the various languages now spoken 
throughout the world ; and if all extinct languages, and 
all intermediate and slowly changing dialects, had to 
be included, such an arrangement would, I think, be 
the only possible one. Yet it might be that some very 
ancient language had altered little, and had given rise 
to few new languages, whilst others (owing to the 
spreading and subsequent isolation and states of civilisa- 
tion of the several races, descended from a common 
race) had altered much, and had given rise to many new 
languages and dialects. The various degrees of differ- 
ence in the languages from the same stock, would have 
to be expressed by groups subordinate to groups ; but 
the proper or even only possible arrangement would still 
be genealogical; and this would be strictly natural, as 


it would connect together all languages, extinct and 
modern, by the closest affinities, and would give the 
filiation and origin of each tongue. 

In confirmation of this view, let us glance at 
the classification of varieties, which are believed or 
known to have descended from one species. These 
are grouped under species, with sub-varieties under 
varieties; and with our domestic productions, several 
other grades of difference are requisite, as we have 
seen with pigeons. The origin of the existence of 
groups subordinate to groups, is the same with varieties 
as with species, namely, closeness of descent with various 
degrees of modification. Nearly the same rules are fol- 
lowed in classifying varieties, as with species. Authors 
have insisted on the necessity of classing varieties on a 
natural instead of an artificial system ; we are cau- 
tioned, for instance, not to class two varieties of the 
pine-apple together, merely because their fruit, though 
the most important part, happens to be nearly identical ; 
no one puts the Swedish and common turnips together, 
though the esculent and thickened stems are so similar. 
Whatever part is found to be most constant, is used 
in classing varieties : thus the great agriculturist Mar- 
shall says the horns are very useful for this purpose 
with cattle, because they are less variable than the 
shape or colour of the body, &c. ; whereas with sheep 
the horns are much less serviceable, because less con- 
stant. In classing varieties, I apprehend if we had a 
real pedigree, a genealogical classification would be 
universally preferred; and it has been attempted by 
some authors. For we might feel sure, whether there 
had been more or less modification, the principle of 
inheritance would keep the forms together which were 
allied in the greatest number of points. In tumbler 
pigeons, though some sub-varieties differ from the others 


in the important character of having a longer beak, yet 
all are kept together from having the common habit 
of tumbling ; but the short-faced breed has nearly or 
quite lost this habit ; nevertheless, without any reasoning 
or thinking on the subject, these tumblers are kept in 
the same group, because allied in blood and alike in 
some other respects. If it could be proved that the 
Hottentot had descended from the Negro, I think he 
would be classed under the Negro group, however much 
he might differ in colour and other important characters 
from negroes. 

With species in a state of nature, every naturalist has 
in fact brought descent into his classification ; for he 
includes in his lowest grade, or that of a species, the 
two sexes ; and how enormously these sometimes differ 
in the most important characters, is known to every 
naturalist : scarcely a single fact can be predicated in 
common of the males and hermaphrodites of certain 
cirripedes, when adult, and yet no one dreams of sepa- 
rating them. The naturalist includes as one species the 
several larval stages of the same individual, however 
much they may differ from each other and from the 
adult; as he likewise includes the so-called alternate 
generations of Steenstrup, which can only in a technical 
sense be considered as the same individual. He in- 
cludes monsters ; he includes varieties, not solely be- 
cause they closely resemble the parent-form, but because 
they are descended from it. He who believes that the 
cowslip is descended from the primrose, or conversely, 
ranks them together as a single species, and gives a 
single definition. As soon as three Orchidean forms 
(Monochanthus, Myanthus, and Catasetum), which had 
previously been ranked as three distinct genera, were 
known to be sometimes produced on the same spike, 
they were immediately included as a single species. 


But it may be asked, what ought we to do, if it could be 
proved that one species of kangaroo had been produced, 
by a long course of modification, from a bear ? Ought 
we to rank this one species with bears, and what should 
we do with the other species? The supposition is of 
course preposterous ; and I might answer by the argu- 
mentum ad hominem, and ask what should be done if a 
perfect kangaroo were seen to come out of the womb of 
a bear? According to all analogy, it would be ranked 
with bears ; but then assuredly all the other species of 
the kangaroo family would have to be classed under 
the bear genus. The whole case is preposterous ; for 
where there has been close descent in common, there 
will certainly be close resemblance or affinity. 

As descent has universally been used in classing to- 
gether the individuals of the same species, though the 
males and females and larvae are sometimes extremely 
different ; and as it has been used in classing varieties 
which have undergone a certain, and sometimes a con- 
siderable amount of modification, may not this same 
element of descent have been unconsciously used in 
grouping species under genera, and genera under higher 
groups, though in these cases the modification has been 
greater in degree, and has taken a longer time to com- 
plete ? I believe it has thus been unconsciously used ; 
and only thus can I understand the several rules and 
guides which have been followed by our best system- 
atists. We have no written pedigrees ; we have to 
make out community of descent by resemblances of any 
kind. Therefore we choose those characters which, as> 
far as we can judge, are the least likely to have been 
modified in relation to the conditions of life to which 
each species has been recently exposed. Eudimentary 
structures on this view are as good as, or even some- 
times better than, other parts of the organisation. We 


care not how trifling a character may be — let it be the 
mere inflection of the angle of the jaw, the manner in 
which an insect's wing is folded, whether the skin be 
covered by hair or feathers — if it prevail throughout 
many and different species, especially those having very 
different habits of life, it assumes high value ; for we 
can account for its presence in so many forms with such 
different habits, only by its inheritance from a common 
parent. We may err in this respect in regard to single 
V points of structure, but when several characters, let 
them be ever so trifling, occur together throughout a 
large group of beings having different habits, we may 
feel almost sure, on the theory of descent, that these 
characters have been inherited from a common ancestor. 
And we know that such correlated or aggregated cha- 
racters have especial value in classification. 

We can understand why a species or a group of spe- 
cies may depart, in several of its most important cha- 
racteristics, from its allies, and yet be safely classed with 
them. This may be safely done, and is often done, as 
long as a sufficient number of characters, let them be 
ever so unimportant, betrays the hidden bond of com- 
munity of descent. Let two forms have not a single 
character in common, yet if these extreme forms are 
connected together by a chain of intermediate groups, 
we may at once infer their community of descent, and 
we put them all into the same class. As we find organs 
of high physiological importance — those which serve to 
preserve life under the most diverse conditions of exist- 
ence — are generally the most constant, we attach espe- 
cial value to them ; but if these same organs, in another 
group or section of a group, are found to differ much, we 
at once value them less in our classification. We shall 
hereafter, I think, clearly see why embryological cha- 
racters are of such high classificatory importance. 


Geographical distribution may sometimes be brought 
usefully into play in classing large and widely-distri- 
buted genera, because all the species of the same genus, 
inhabiting any distinct and isolated region, have in all 
probability descended from the same parents. 

We can understand, on these views, the very im- 
portant distinction between real affinities and analogical 
or adaptive resemblances. Lamarck first called atten- 
tion to this distinction, and he has been ably followed 
by Macleay and others. The resemblance, in the shape 
of the body and in the fin-like anterior limbs, between 
the dugong, which is a pachydermatous animal, and the 
whale, and between both these mammals and fishes, is 
analogical. Amongst insects there are innumerable in- 
stances : thus Linnaeus, misled by external appearances, 
actually classed an homopterous insect as a moth. We 
see something of the same kind even in our domestic 
varieties, as in the thickened stems of the common and 
Swedish turnip. The resemblance of the greyhound and 
racehorse is hardly more fanciful than the analogies 
winch have been drawn by some authors between very r 
distinct animals. On my view of characters being of 
real importance for classification, only in so far as they 
reveal descent, we can clearly understand why analogical 
or adaptive character, although of the utmost importance 
to the welfare of the being, are almost valueless to the 
systematist. For animals, belonging to two most distinct 
lines of descent, may readily become adapted to similar 
conditions, and thus assume a close external resem- 
blance ; but such resemblances will not reveal — will 
rather tend to conceal their blood-relationship to their 
proper lines of descent. We can also understand the 
apparent paradox, that the very same characters arc 
analogical when one class or order is compared with 
another, but give true affinities when the members of 


the same class or order are compared one with another : 
thus the shape of the body and fin-like limbs are only 
analogical when whales are compared with fishes, being 
adaptations in both classes for swimming through the 
water; but the shape of the body and fin-like limbs 
serve as characters exhibiting true affinity between the 
several members of the whale family ; for these ceta- 
ceans agree in so many characters, great and small, 
that we cannot doubt that they have inherited their 
general shape of body and structure of limbs from a 
common ancestor. So it is with fishes. 

As members of distinct classes have often been 
adapted by successive slight modifications to live under 
nearly similar circumstances, — to inhabit for instance 
the three elements of land, air, and water, — we can per- 
haps understand how it is that a numerical parallelism 
has sometimes been observed between the sub-groups 
in distinct classes. A naturalist, struck by a parallelism 
of this nature in any one class, by arbitrarily raising 
or sinking the value of the groups in other classes 
(and all our experience shows that this valuation 
has hitherto been arbitrary), could easily extend the 
parallelism over a wide range ; and thus the septenary, 
quinary, quaternary, and ternary classifications have 
probably arisen. 

As the modified descendants of dominant species, 
belonging to the larger genera, tend to inherit the 
advantages, which made the groups *to which they belong 
large and their parents dominant, they are almost sure 
to spread widely, and to seize on more and more places in 
the economy of nature. The larger and more dominant 
groups thus tend to go on increasing in size ; and they 
consequently supplant many smaller and feebler groups. 
Thus we can account for the fact that all organisms, 
recent and extinct, are included under a few great 


orders, under still fewer classes, and all in one great 
natural system. As showing how few the higher groups 
are in number, and how widely spread they are through- 
out the world, the fact is striking, that the discovery of 
Australia has not added a single insect belonging to a 
new order ; and that in the vegetable kingdom, as I 
learn from Dr. Hooker, it has added only two or three 
orders of small size. 

In the chapter on geological succession I attempted 
to show, on the principle of each group having generally 
diverged much in character during the long-continued 
process of modification, how it is that the more ancient 
forms of life often present characters in some slight 
degree intermediate between existing groups. A few 
old and intermediate parent-forms having occasionally 
transmitted to the present day descendants but little 
modified, will give to us our so-called osculant or aber- 
rant groups. The more aberrant any form is, the 
greater must be the number of connecting forms which 
on my theory have been exterminated and utterly lost. 
And we have some evidence of aberrant forms having 
suffered severely from extinction, for they are gene- 
rally represented by extremely few species; and such 
species as do occur are generally very distinct from 
each other, which again implies extinction. The genera 
Ornithorhynchus and Lepidosiren, for example, would 
not have been less aberrant had each been represented 
by a dozen species instead of by a single one ; but such 
richness in species, as I find after some investigation, 
does not commonly fall to the lot of aberrant genera. 
We can, I think, account for this fact only by looking 
at aberrant forms as failing groups conquered by more 
successful competitors, with a few members preserved by 
some unusual coincidence of favourable circumstances. 

Mr. Waterhouse has remarked that, when a member 


belonging to one group of animals exhibits an affinity 
to a quite distinct group, this affinity in most cases is 
general and not special : thus, according to Mr. Water- 
house, of all Kodents, the bizcacha is most nearly related 
to Marsupials ; but in the points in which it approaches 
this order, its relations are general, and not to any one 
marsupial species more than to another. As the points 
of affinity of the bizcacha to Marsupials are believed 
to be real and not merely adaptive, they are due on 
my theory to inheritance in common. Therefore we 
must suppose either that all Eodents, including the biz- 
cacha, branched off from some very ancient Marsupial, 
which will have had a character in some degree inter- 
mediate with respect to all existing Marsupials ; or 
that both Eodents and Marsupials branched off from a 
common progenitor, and that both groups have since 
undergone much modification in divergent directions. 
On either view we may suppose that the bizcacha has 
retained, by inheritance, more of the character of its 
ancient progenitor than have other Eodents ; and 
therefore it will not be specially related to any one 
existing Marsupial, but indirectly to all or nearly all 
Marsupials, from having partially retained the character 
of their common progenitor, or of an early member of 
the group. On the other hand, of all Marsupials, as 
Mr. Waterhouse has remarked, the phascolomys re- 
sembles most nearly, not any one species, but the 
general order of Eodents. In this case, however, it 
may be strongly suspected that the resemblance is only 
analogical, owing to the phascolomys having become 
adapted to habits like those of a Eodent. The elder 
De Candolle has made nearly similar observations on the 
general nature of the affinities of distinct orders of plants. 
On the principle of the multiplication and gradual 
divergence in character of the species descended from 


a common parent, together with then retention by 
inheritance of some characters in common, we can 
understand the excessively complex and radiating 
affinities by which all the members of the same family 
or higher group are connected together. For the com- 
mon parent of a whole family of species, now broken 
up by extinction into distinct groups and sub-groups, will 
have transmitted some of its characters, modified in 
various ways and degrees, to all ; and the several 
species will consequently be related to each other by 
circuitous lines of affinity of various lengths (as may be 
seen in the diagram so often referred to), mounting up 
through many predecessors. As it is difficult to show 
the blood-relationship between the numerous kindred 
of any ancient and noble family, even by the aid of a 
genealogical tree, and almost impossible to do this 
without this aid, we can understand the extraordinary 
difficulty which naturalists have experienced in describ- 
ing, without the aid of a diagram, the various affinities 
which they perceive between the many living and ex- 
tinct members of the same great natural class. 

Extinction, as we have seen in the fourth chapter, 
has played an important part in defining and widening 
the intervals between the several groups in each class. 
We may thus account even for the distinctness of 
whole classes from each other — for instance, of birds 
from all other vertebrate animals — by the belief that 
many ancient forms of life have been utterly lost, 
through winch the early progenitors of birds were 
formerly connected with the early progenitors of the 
other vertebrate classes. There has been less entire 
extinction of the forms of life which once connected 
fishes with batrachians. There has been still less in 
some other classes, as in that of the Crustacea, for 
here the most wonderfully diverse forms are still tied 


together by a long, but broken, chain of affinities. 
Extinction has only separated groups : it has by no 
means made them ; for if every form which has ever 
lived on this earth were suddenly to reappear, though it 
would be quite impossible to give definitions by which 
each group could be distinguished from other groups, as 
all would blend together by steps as fine as those be- 
tween the finest existing varieties, nevertheless a natural 
classification, or at least a natural arrangement, would be 
possible. We shall see this by turning to the diagram : 
the letters, A to L, may represent eleven Silurian genera, 
some of which have produced large groups of modified 
descendants. Every intermediate link between these 
eleven genera and their primordial parent, and every 
intermediate link in each branch and sub-branch of 
their descendants, may be supposed to be still alive ; 
and the links to be as fine as those between the finest 
varieties. In this case it would be quite impossible to 
give any definition by which the several members of the 
several groups could be distinguished from their more 
immediate parents ; or these parents from their ancient 
and unknown progenitor. Yet the natural arrangement 
in the diagram would still hold good ; and, on the prin- 
ciple of inheritance, all the forms descended from A, or 
from I, would have something in common. In a tree we 
can specify this or that branch, though at the actual 
fork the two unite and blend together. We could not, 
as I have said, define the several groups ; but we could 
pick out types, or forms, representing most of the cha- 
racters of each group, whether large or small, and thus 
give a general idea of the value of the differences 
between them. This is what we should be driven to, if 
we were ever to succeed in collecting all the forms 
in any class which have lived throughout all time and 
space. We shall certainly never succeed in making 


so perfect a collection : nevertheless, in certain classes, 
we are tending in this direction ; and Milne Edwards 
has lately insisted, in an able paper, on the high import- 
ance of looking to types, whether or not we can separate 
and define the groups to which such types belong. 

Finally, we have seen that natural selection, which 
results from the struggle for existence, and which almost 
inevitably induces extinction and divergence of character 
in the many descendants from one dominant parent- 
species, explains that great and universal feature in the 
affinities of all organic beings, namely, their subordina- 
tion in group under group. We use the element of 
descent in classing the individuals of both sexes and of 
all ages, although having few characters in common, 
under one species ; we use descent in classing acknow- 
ledged varieties, however different they may be from their 
parent ; and I believe this element of descent is the hid- 
den bond of connexion which naturalists have sought 
under the term of the Natural System. On this idea of 
the natural system being, in so far as it has been perfected, 
genealogical in its arrangement, with the grades of differ- 
ence between the descendants from a common parent, 
expressed by the terms genera, families, orders, &c, we 
can understand the rules which we are compelled to 
follow in our classification. AVe can understand why we 
value certain resemblances far more than others ; why 
we are permitted to use rudimentary and useless organs, 
or others of trifling physiological importance ; why, in 
comparing one group with a distinct group, we summarily 
reject analogical or adaptive characters, and yet use these 
same characters within the limits of the same group. 
We can clearly see how it is that all living and extinct 
forms can be grouped together in one great system ; 
and how the several members of each class are con- 
nected together by the most complex and radiating 



/ lines of affinities. We shall never, probably, disen- 
tangle the inextricable web of affinities between the 
members of any one class ; but when we have a dis- 
tinct object in view, and do not look to some unknown 
plan of creation, we may hope to make sure but slow 

Morphology. — We have seen that the members of 
the same class, independently of their habits of life, 
resemble each other in the general plan of their organ- 
isation. This resemblance is often expressed by the 
term " unity of type ;" or by saying that the several 
. parts and organs in the different species of the class 
are homologous. The whole subject is included under 
the general name of Morphology. This is the most 
interesting department of natural history, and may 
be said to be its very soul. What can be more curious 
than that the hand of a man, formed for grasping, 
that of a mole for digging, the leg of the horse, the 
paddle of the porpoise, and the wing of the bat, should 
all be constructed on the same pattern, and should 
include the same bones, in the same relative positions? 
Geoffroy St. Hilaire has insisted strongly on the high 
importance of relative connexion in homologous organs : 
the parts may change to almost any extent in form and 
size, and yet they always remain connected together 
in the same order. We never find, for instance, the 
bones of the arm and forearm, or of the thigh and leg, 
transposed. Hence the same names can be given to 
the homologous bones in widely different animals. We 
see the same great law in the construction of the mouths 
of insects: what can be more different than the im- 
mensely long spiral proboscis of a sphinx-moth, the 
curious folded one of a bee or bug, and the great jaws 
of a beetle ? — yet all these organs, serving for such dif- 


ferent purposes, are formed by infinitely numerous modi- 
fications of an upper lip, mandibles, and two pairs of 
maxilla?. Analogous laws govern the construction of 
the mouths and limbs of crustaceans. So it is with the 
flowers of plants. 

Nothing can be more hopeless than to attempt to 
explain this similarity of pattern in members of the same 
class, by utility or by the doctrine of final causes. The 
hopelessness of the attempt has been expressly admitted 
by Owen in his most interesting work on the ' Nature of 
Limbs.' On the ordinary view of the independent creation 
of each being, we can only say that so it is ; — that it has 
so pleased the Creator to construct each animal and plant. 

The explanation is manifest on the theory of the 
natural selection of successive slight modifications, — 
each modification being profitable in some way to the 
modified form, but often affecting by correlation of 
growth other parts of the organisation. In changes 
of this nature, there will be little or no tendency to 
modify the original pattern, or to transpose parts. The 
bones of a limb might be shortened and widened to any 
extent, and become gradually enveloped in thick mem- 
brane, so as to serve as a fin ; or a webbed foot might 
have all its bones, or certain bones, lengthened to any 
extent, and the membrane connecting them increased 
to any extent, so as to serve as a wing : yet in all this 
great amount of modification there will be no tendency 
to alter the framework of bones or the relative con- 
nexion of the several parts. If we suppose that the 
ancient progenitor, the archetype as it may be called, of 
all mammals, had its limbs constructed on the existing 
general pattern, for whatever purpose they served, we 
can at once perceive the plain signification of the homo- 
logous construction of the limbs throughout the whole 
class. So with the mouths of insects, we have only to 

u 2 


suppose that their common progenitor had an upper lip, 
mandibles, and two pair of maxillae, these parts being 
perhaps very simple in form ; and then natural selection 
will account for the infinite diversity in structure and 
function of the mouths of insects. Nevertheless, it is 
conceivable that the general pattern of an organ might 
become so much obscured as to be finally lost, by the 
atrophy and ultimately by the complete abortion of cer- 
tain parts, by the soldering together of other parts, and 
by the doubling or multiplication of others, — variations 
which we know to be within the limits of possibility. 
In the paddles of the extinct gigantic sea-lizards, and 
in the mouths of certain suctorial crustaceans, the 
general pattern seems to have been thus to a certain 
extent obscured. 

There is another and equally curious branch of the 
present subject ; namely, the comparison not of the same 
part in different members of a class, but of the different 
parts or organs in the same individual. Most physio- 
logists believe that the bones of the skull are homo- 
logous with — that is correspond in number and in re- 
lative connexion with — the elemental parts of a certain 
number of vertebrae. The anterior and posterior limbs 
in each member of the vertebrate and articulate 
classes are plainly homologous. We see the same law in 
comparing the wonderfully complex jaws and legs in 
crustaceans. It is familiar to almost every one, that in a 
flower the relative position of the sepals, petals, stamens, 
and pistils, as well as their intimate structure, are intel- 
ligible on the view that they consist of metamorphosed 
leaves, arranged in a spire. In monstrous plants, we 
often get direct evidence of the possibility of one organ 
being transformed into another; and we can actually 
see in embryonic crustaceans and in many other ani- 
mals, and in flowers, that organs, which when mature 


become extremely different, are at an early stage of 
growth exactly alike. 

How inexplicable are these facts on the ordinary 
view of creation ! Why should the brain be enclosed 
in a box composed of such numerous and such extra- 
ordinarily shaped pieces of bone? As Owen has re- 
marked, the benefit derived from the yielding of the 
separate pieces in the act of parturition of mammals, will 
by no means explain the same construction in the skulls 
of birds. Why should similar bones have been created 
in the formation of the wing and leg of a bat, used as 
they are for such totally different purposes? Why 
should one crustacean, which has an extremely complex 
mouth formed of many parts, consequently always have 
fewer legs ; or conversely, those with many legs have 
simpler mouths? Why should the sepals, petals, sta- 
mens, and pistils in any individual flower, though fitted 
for such widely different purposes, be all constructed 
on the same pattern ? 

On the theory of natural selection, we can satisfactorily 
answer these questions. In the vertebrata, we see a series 
of internal vertebrae bearing certain processes and appen- 
dages ; in the articulata, we see the body divided into a 
series of segments, bearing external appendages ; and in 
flowering plants, we see a series of successive spiral 
whorls of leaves. An indefinite repetition of the same 
part or organ is the common characteristic (as Owen 
has observed) of all low or little-modified forms ; there- 
fore we may readily believe that the unknown progenitor 
of the vertebrata possessed many vertebra? ; the unknown 
progenitor of the articulata, many segments ; and the 
unknown progenitor of flowering plants, many spiral 
whorls of leaves. We have formerly seen that parts many 
times repeated are eminently liable to vary in number 
and structure; consequently it is quite probable that 


natural selection, during a long-continued course of modi- 
fication, should have seized on a certain number of the 
primordially similar elements, many times repeated, and 
have adapted them to the most diverse purposes. And 
as the whole amount of modification will have been 
effected by slight successive steps, we need not wonder 
at discovering in such parts or organs, a certain degree 
of fundamental resemblance, retained by the strong 
principle of inheritance. 

In the great class of molluscs, though we can homo- 
logise the parts of one species with those of another and 
distinct species, we can indicate but few serial homo- 
logies ; that is, we are seldom enabled to say that one 
part or organ is homologous with another in the same 
individual. And we can understand this fact; for in 
molluscs, even in the lowest members of the class, we 
do not find nearly so much indefinite repetition of any 
one part, as we find in the other great classes of the ani- 
mal and vegetable kingdoms. 

Naturalists frequently speak of the skull as formed of 
metamorphosed vertebrae : the jaws of crabs as meta- 
morphosed legs ; the stamens and pistils of flowers as 
metamorphosed leaves ; but it would in these cases pro- 
bably be more correct, as Professor Huxley has remarked, 
to speak of both skull and vertebra?, both jaws and legs, 
&c, — as having been metamorphosed, not one from the 
other, but from some common element. Naturalists, 
however, use such language only in a metaphorical 
sense : they are far from meaning that during a long 
course of descent, primordial organs of any kind — verte- 
bra? in the one case and legs in the other — have actually 
been modified into skulls or jaws. Yet so strong is the 
appearance of a modification of this nature having oc- 
curred, that naturalists can hardly avoid employing 
language having this plain signification. On my view 


these terms may be used literally ; and the wonderful 
fact of the jaws, for instance, of a crab retaining nume- 
rous characters, which they would probably have retained 
through inheritance, if they had really been metamor- 
phosed during a long course of descent from true legs, 
or from some simple appendage, is explained. 

Embryology. — It has already been casually remarked 
that certain organs in the individual, which when mature 
become widely different and serve for different purposes, 
are in the embryo exactly alike. The embryos, also, of 
distinct animals within the same class are often strikingly 
similar : a better proof of this cannot be given, than a k 
circumstance mentioned by Agassiz, namely, that having 
forgotten to ticket the embryo of some vertebrate ani- 
mal, he cannot now tell whether it be that of a mammal, 
bird, or reptile. The vermiform larvae of moths, flies, 
beetles, &c, resemble each other much more closely 
than do the mature insects ; but in the case of larva?, 
the embryos are active, and have been adapted for spe- 
cial lines of life. A trace of the law of embryonic re- 
semblance, sometimes lasts till a rather late age : thus 
birds of the same genus, and of closely allied genera, 
often resemble each other in their first and second 
plumage; as we see in the spotted feathers in the 
thrush group. In the cat tribe, most of the species are 
striped or spotted in lines ; and stripes can be plainly 
distinguished in the whelp of the lion. We occasion- 
ally though rarely see something of tins kind in plants : 
thus the embryonic leaves of the ulex or furze, and the 
first leaves of the phyllodineous acacias, are pinnate or i 
divided like the ordinary leaves of the leguminosae. 

The points of structure, in which the embryos of 
widely different animals of the same class resemble 
each other, often have no direct relation to their condi- 


tions of existence. We cannot, for instance, suppose 
that in the embryos of the vertebrata the peculiar 
loop-like course of the arteries near the branchial slits 
are related to similar conditions, — in the young mammal 
which is nourished in the womb of its mother, in the 
egg of the bird which is hatched in a nest, and in the 
spawn of a frog under water. We have no more reason 
to believe in such a relation, than we have to believe 
that the same bones in the hand of a man, wing of a 
bat, and fin of a porpoise, are related to similar condi- 
tions of life. No one will suppose that the stripes on 
the whelp of a lion, or the spots on the young blackbird, 
are of any use to these animals, or are related to the 
conditions to which they are exposed. 

The case, however, is different when an animal during 
any part of its embryonic career is active, and has to 
provide for itself. The period of activity may come on 
earlier or later in life ; but whenever it comes on, the 
adaptation of the larva to its conditions of life is just as 
perfect and as beautiful as in the adult animal. From 
such special adaptations, the similarity of the larva? or 
active embryos of allied animals is sometimes much ob- 
scured ; and cases could be given of the larvae of two 
species, or of two groups of species, differing quite as 
much, or even more, from each other than do their adult 
parents. In most cases, however, the larvae, though 
active, still obey more or less closely the law of com- 
mon embryonic resemblance. Cirripedes afford a good 
instance of this : even the illustrious Cuvier did not per- 
ceive that a barnacle was, as it certainly is, a crustacean ; 
but a glance at the larva shows this to be the case in an 
unmistakeable manner. So again the two main divi- 
sions of cirripedes, the pedunculated and sessile, which 
differ widely in external appearance, have larvae in all 
their several stages barely distinguishable. 


The embryo in the course of development generally 
rises in organisation: I use this expression, though I 
am aware that it is hardly possible to define clearly 
what is meant by the organisation being higher or 
lower. But no one probably will dispute that the but- 
terfly is higher than the caterpillar. In some cases, 
however, the mature animal is generally considered as 
lower in the scale than the larva, as with certain para- 
sitic crustaceans. To refer once again to cirripedes : 
the larvae in the first stage have three pairs of legs, a 
very simple single eye, and a probosciformed mouth, 
with which they feed largely, for they increase much in 
size. In the second stage, answering to the chrysalis 
stage of butterflies, they have six pairs of beautifully 
constructed natatory legs, a pair of magnificent com- 
pound eyes, and extremely complex antennae ; but they 
have a closed and imperfect mouth, and cannot feed : 
their function at this stage is, to search by their well- 
developed organs of sense, and to reach by their active 
powers of swimming, a proper place on which to become 
attached and to undergo their final metamorphosis. 
When this is completed they are fixed for life : their legs 
are now converted into prehensile organs; they again 
obtain a well-constructed mouth ; but they have no an- 
tennae, and their two eyes are now reconverted into a 
minute, single, and very simple eye-spot. In this last and 
complete state, cirripedes may be considered as either 
more highly or more lowly organised than they were in the 
larval condition. Bat in some genera the larvae become 
developed either into hermaphrodites having the ordi- 
nary structure, or into what I have called complemental 
males : and in the latter, the development has assuredly 
been retrograde ; for the male is a mere sack, which lives 
for a short time, and is destitute of mouth, stomach, or 
other organ of importance, excepting for reproduction. 

u 3 


We are so much accustomed to see differences in 
structure between the embryo and the adult, and like- 
wise a close similarity in the embryos of widely different 
animals within the same class, that we might be led 
to look at these facts as necessarily contingent in some 
manner on growth. But there is no obvious reason why, 
for instance, the wing of a bat, or the fin of a porpoise, 
should not have been sketched out with all the parts 
in proper proportion, as soon as any structure became 
visible in the embryo. And in some whole groups of 
animals and in certain members of other groups, the 
embryo does not at any period differ widely from the 
adult : thus Owen has remarked in regard to cuttle-fish, 
" there is no metamorphosis ; the cephalopodic character 
is manifested long before the parts of the embryo are 
completed ;" and again in spiders, " there is nothing 
worthy to be called a metamorphosis." The larvae of 
insects, whether adapted to the most diverse and active 
habits, or quite inactive, being fed by their parents or 
placed in the midst of proper nutriment, yet nearly all 
pass through a similar worm-like stage of development ; 
but in some few cases, as in that of Aphis, if we look to 
the admirable drawings by Professor Huxley of the 
development of this insect, we see no trace of the ver- 
miform stage. 

How, then, can we explain these several facts in 
embryology, — namely the very general, but not uni- 
versal difference in structure between the embryo and 
the adult ; — of parts in the same indivividual embryo, 
which ultimately become very unlike and serve for 
diverse purposes, being at tins early period of growth 
alike ; — of embryos of different species within the same 
class, generally, but not universally, resembling each 
other ; — of the structure of the embryo not being closely 
related to its conditions of existence, except when the 


embryo becomes at any period of life active and has to 
provide for itself; — of tlie embryo apparently having 
sometimes a higher organisation than the mature 
animal, into which it is developed. I believe that all 
these facts can be explained, as follows, on the view of 
descent with modification. 

It is commonly assumed, perhaps from monstrosities 
often affecting the embyro at a very early period, 
that slight variations necessarily appear at an equally 
early period. But we have little evidence on this head — 
indeed the evidence rather points the other way ; for it 
is notorious that breeders of cattle, horses, and various 
fancy animals, cannot positively tell, until some time 
after the animal has been born, what its merits or form 
will ultimately turn out. "We see this plainly in our own 
children ; we cannot always tell whether the child will 
be tall or short, or what its precise features will be. The 
question is not, at what period of life any variation has 
been caused, but at what period it is fully displayed. 
The cause may have acted, and I believe generally has 
acted, even before the embryo is formed ; and the varia- 
tion may be due to the male and female sexual elements 
having been affected by the conditions to which either 
parent, or their ancestors, have been exposed. Never- 
theless an effect thus caused at a very early period, even 
before the formation of the embryo, may appear late in 
life ; as when an hereditary disease, which appears in 
old age alone, has been communicated to the offspring 
from the reproductive element of one parent. Or 
again, as when the horns of cross-bred cattle have been 
affected by the shape of the horns of either parent. For 
the welfare of a very young animal, as long as it remains 
in its mother's womb, or in the egg, or as long as it is 
nourished and protected by its parent, it must be quite 
unimportant whether most of its characters are fully 


acquired a little earlier or later in life. It would 
not signify, for instance, to a bird which obtained its 
food best by having a long beak, whether or not it 
assumed a beak of this particular length, as long as it 
was fed by its parents. Hence, I conclude, that it is 
quite possible, that each of the many successive modi- 
fications, by which each species has acquired its pre- 
sent structure, may have supervened at a not very 
early period of life ; and some direct evidence from our 
domestic animals supports this view. But in other cases 
it is quite possible that each successive modification, or 
most of them, may have appeared at an extremely early 

I have stated in the first chapter, that there is some 
evidence to render it probable, that at whatever age any 
variation first appears in the parent, it tends to reappear 
at a corresponding age in the offspring. Certain varia- 
tions can only appear at corresponding ages, for in- 
stance, peculiarities in the caterpillar, cocoon, or imago 
states of the silk-moth ; or, again, in the horns of almost 
full-grown cattle. But further than this, variations which, 
for all that we can see, might have appeared earlier or 
later in life, tend to appear at a corresponding age in 
the offspring and parent. I am far from meaning that 
this is invariably the case ; and I could give a good 
many cases of variations (taking the word in the largest 
sense) winch have supervened at an earlier age in the 
child than in the parent. 

These two principles, if then truth be admitted, will, 
I believe, explain all the above specified leading facts 
in embryology. But first let us look at a few analogous 
cases in domestic varieties. Some authors who have 
written on Dogs, maintain that the greyhound and bull- 
dog, though appearing so different, are really varieties 
most closely allied, and have probably descended from 


the same wild stock ; hence I was curious to see how far 
their puppies differed from each other : I was told by 
breeders that they differed just as much as their parents, 
and this, judging by the eye, seemed almost to be the 
case ; but on actually measuring the old dogs and their 
six-clays old puppies, I found that the puppies had not 
nearly acquired their full amount of proportional differ- 
ence. So, again, I was told that the foals of cart and 
race-horses differed as much as the full-grown animals ; 
and this surprised me greatly, as I think it probable that 
the difference between these two breeds has been wholly 
caused by selection under domestication ; but having 
had careful measurements made of the dam and of a 
three-days old colt of a race and heavy cart-horse, I find 
that the colts have by no means acquired their full 
amount of proportional difference. 

As the evidence appears to me conclusive, that the 
several domestic breeds of Pigeon have descended from 
one wild species, I compared young pigeons of various 
breeds, within twelve hours after being hatched ; I care- 
fully measured the proportions (but will not here give 
details) of the beak, width of mouth, length of nostril 
and of eyelid, size of feet and length of leg, in the 
wild stock, in pouters, fantails, runts, barbs, dragons, 
carriers, and tumblers. Now some of these birds, when 
mature, differ so extraordinarily in length and form 
of beak, that they would, I cannot doubt, be ranked in 
distinct genera, had they been natural productions. But 
when the nestling birds of these several breeds were 
placed in a row, though most of them could be distin- 
guished from each other, yet their proportional differ- 
ences in the above specified several points were in- 
comparably less than in the full-grown birds. Some 
characteristic points of difference — for instance, that of 
the width of mouth — could hardly be detected in the 


young. But there was one remarkable exception to this 
rule, for the young of the short-faced tumbler differed 
from the young of the wild rock-pigeon and of the other 
breeds, in all its proportions, almost exactly as much as 
in the adult state. 

The two principles above given seem to me to explain 
these facts in regard to the later embryonic stages of 
our domestic varieties. Fanciers select their horses, 
dogs, and pigeons, for breeding, when they are nearly 
grown up : they are indifferent whether the desired 
qualities and structures have been acquired earlier or 
later in life, if the full-grown animal possesses them. 
And the cases just given, more especially that of 
pigeons, seem to show that the characteristic differences 
which give value to each breed, and which have been 
accumulated by man's selection, have not generally first 
appeared at an early period of life, and have been in- 
herited by the offspring at a corresponding not early 
period. But the case of the short-faced tumbler, which 
when twelve hours old had acquired its proper propor- 
tions, proves that this is not the universal rule ; for here 
the characteristic differences must either have appeared 
at an earlier period than usual, or, if not so, the differ- 
ences must have been inherited, not at the corresponding, 
but at an earlier age. 

Now let us apply these facts and the above two 
principles — which latter, though not proved true, can 
be shown to be in some degree probable — to species 
in a state of nature. Let us take a genus of birds, 
descended on my theory from some one parent-species, 
and of which the several new species have become 
modified through natural selection in accordance with 
their diverse habits. Then, from the many slight suc- 
cessive steps of variation having supervened at a rather 
late age, and having been inherited at a corresponding 


age, the young of the new species of our supposed genus 
will manifestly tend to resemble each other much more 
closely than do the adults, just as we have seen in the 
case of pigeons. We may extend this view to whole 
families or even classes. The fore-limbs, for instance, 
which served as legs in the parent-species, may be- 
come, by a long course of modification, adapted in one 
descendant to act as hands, in another as paddles, in 
another as wings ; and on the above two principles — 
namely of each successive modification supervening at 
a rather late age, and being inherited at a corre- 
sponding late age — the fore-limbs in the embryos of 
the several descendants of the parent-species will still 
resemble each other closely, for they will not have 
been modified. But in each individual new species, 
the embryonic fore-limbs will differ greatly from the 
fore-limbs in the mature animal; the limbs in the 
latter having undergone much modification at a 
rather late period of life, and having thus been con- 
verted into hands, or paddles, or wings. Whatever 
influence long-continued exercise or use on the one 
hand, and disuse on the other, may have in modi- 
fying an organ, such influence will mainly affect the 
mature animal, which has come to its full powers of 
activity and has to gain its own living ; and the effects 
thus produced will be inherited at a corresponding 
mature age. Whereas the young will remain unmodified, 
or be modified in a lesser degree, by the effects of use 
and disuse. 

In certain cases the successive steps of variation 
might supervene, from causes of which we are wholly 
ignorant, at a very early period of life, or each step 
might be inherited at an earlier period than that at 
which it first appeared. In either case (as with the 
short-faced tumbler) the young or embryo would closely 


resemble the mature parent-form. We have seen that 
this is the rule of development in certain whole groups 
of animals, as with cuttle-fish and spiders, and with a 
few members of the great class of insects, as with Aphis. 
With respect to the final cause of the young in these 
cases not undergoing any metamorphosis, or closely 
resembling their parents from their earliest age, we 
can see that this would result from the two following 
contingencies ; firstly, from the young, during a course 
of modification carried on for many generations, having 
to provide for their own wants at a very early stage 
of development, and secondly, from their following 
exactly the same habits of life with their parents ; for 
in this case, it would be indispensable for the existence 
of the species, that the child should be modified at a 
very early age in the same manner with its parents, in 
accordance with their similar habits. Some further 
explanation, however, of the embryo not undergoing 
any metamorphosis is perhaps requisite. If, on the other 
hand, it profited the young to follow habits of life in any 
degree different from those of their parent, and conse- 
quently to be constructed in a slightly different manner, 
then, on the principle of inheritance at corresponding- 
ages, the active young or larvae might easily be ren- 
dered by natural selection different to any conceivable 
extent from their parents. Such differences might, 
also, become correlated with successive stages of deve- 
lopment; so that the larvae, in the first stage, might 
differ greatly from the larvae in the second stage, as we 
have seen to be the case with cirripedes. The adult 
might become fitted for sites or habits, in which organs 
of locomotion or of the senses, &c, would be useless ; 
and in this case the final metamorphosis would be said 
to be retrograde. 

As all the organic beings, extinct and recent, which 


have ever lived on this earth have to be classed together, 
and as all have been connected by the finest gradations, 
the best, or indeed, if our collections were nearly perfect, 
the only possible arrangement, would be genealogical. 
Descent being on my view the hidden bond of con- 
nexion which naturalists have been seeking under 
the term of the natural system. On this view we 
can understand how it is that, in the eyes of most 
naturalists, the structure of the embryo is even more 
important for classification than that of the adult. For 
the embryo is the animal in its less modified state ; and 
in so far it reveals the structure of its progenitor. 
In two groups of animal, however much they may at 
present differ from each other in structure and habits, if 
they pass through the same or similar embryonic stages, 
we may feel assured that they have both descended 
from the same or nearly similar parents, and are there- 
fore in that degree closely related. Thus, community in 
embryonic structure reveals community of descent. It 
will reveal this community of descent, however much 
the structure of the adult may have been modified and 
obscured ; we have seen, for instance, that cirripedes 
can at once be recognised by their larvae as belonging 
to the great class of crustaceans. As the embryonic 
state of each species and group of species partially shows 
us the structure of their less modified ancient progeni- 
tors, we can clearly see why ancient and extinct forms 
of life should resemble the embryos of their descend- 
ants, — our existing species. Agassiz believes this to 
be a law of nature ; but I am bound to confess that I 
only hope to see the law hereafter proved true. It can 
be proved true in those cases alone in which the ancient 
state, now supposed to be represented in many embryos, 
has not been obliterated, either by the successive varia- 
tions in a long course of modification having super- 



vened at a very early age, or by the variations having 
been inherited at an earlier period than that at which 
they first appeared. It should also be borne in mind, 
that the supposed law of resemblance of ancient forms 
of life to the embryonic stages of recent forms, may be 
true, but yet, owing to the geological record not extend- 
ing far enough back in time, may remain for a long 
period, or for ever, incapable of demonstration. 

Thus, as it seems to me, the leading facts in embryo- 
logy, which are second in importance to none in natural 
history, are explained on the principle of slight modifi- 
cations not appearing, in the many descendants from 
some one ancient progenitor, at a very early period in 
the life of each, though perhaps caused at the earliest, 
and being inherited at a corresponding not early 
period. Embryology rises greatly in interest, when 
we thus look at the embryo as a picture, more or less 
obscured, of the common parent-form of each great class 
of animals. 

Rudimentary, atrophied, or aborted organs. — Organs 
or parts in this strange condition, bearing the stamp of 
inutility, are extremely common throughout nature. For 
instance, rudimentary mamma? are very general in the 
males of mammals : I presume that the " bastard-wing " 
in birds may be safely considered as a digit in a rudi- 
mentary state : in very many snakes one lobe of the lungs 
is rudimentary ; in other snakes there are rudiments 
of the pelvis and hind limbs. Some of the cases of rudi- 
mentary organs are extremely curious ; for instance, the 
presence of teeth in foetal whales, which when grown 
up have not a tooth in their heads ; and the presence of 
teeth, which never cut through the gums, in the upper 
jaws of our unborn calves. It has even been stated on 
good authority that rudiments of teetli can be detected 


in the beaks of certain embryonic birds. Nothing can 
be plainer than that wings are formed for flight, yet in 
how many insects do we see wings so reduced in size as 
to be utterly incapable of flight, and not rarely lying 
under wing-cases, firmly soldered together ! 

The meaning of rudimentary organs is often quite 
unmistakeable : for instance there are beetles of the 
same genus (and even of the same species) resembling 
each other most closely in all respects, one of which will 
have full-sized wings, and another mere rudiments of 
membrane ; and here it is impossible to doubt, that the 
rudiments represent wings. Rudimentary organs some- 
times retain their potentiality, and are merely not deve- 
loped : this seems to be the case with the mammae of 
male mammals, for many instances are on record of 
these organs having become well developed in full-grown 
males, and having secreted milk. So again there are 
normally four developed and two rudimentary teats in 
the udders of the genus Bos, but in our domestic cows 
the two sometimes become developed and give milk. In 
individual plants of the same species the petals some- 
times occur as mere rudiments, and sometimes in a well- 
developed state. In plants with separated sexes, the 
male flowers often have a rudiment of a pistil; and 
Kolreuter found that by crossing such male plants with 
an hermaphrodite species, the rudiment of the pistil in 
the hybrid offspring was much increased in size ; and 
this shows that the rudiment and the perfect pistil are 
essentially alike in nature. 

An organ serving for two purposes, may become rudi- 
mentary or utterly aborted for one, even the more 
important purpose ; and remain perfectly efficient for 
the other. Thus in plants, the office of the pistil is to 
allow the pollen-tubes to reach the ovules protected in 
the ovarium at its base. The pistil consists of a stigma 


supported on the style ; but in some Compositae, the 
male florets, which of course cannot be fecundated, have 
a pistil, which is in a rudimentary state, for it is not 
crowned with a stigma ; but the style remains well de- 
veloped, and is clothed with hairs as in other compo- 
sitae, for the purpose of brushing the pollen out of the 
surrounding anthers. Again, an organ may become 
rudimentary for its proper purpose, and be used for a 
distinct object : in certain fish the swim-bladder seems 
to be rudimentary for its proper function of giving buoy- 
ancy, but has become converted into a nascent breathing 
organ or lung. Other similar instances could be given. 

Kudimentary organs in the individuals of the same 
species are very liable to vary in degree of development 
and in other respects. Moreover, in closely allied 
species, the degree to which the same organ has 
been rendered rudimentary occasionally differs much. 
This latter fact is well exemplified in the state of the 
wiugs of the female moths in certain groups. Kudi- 
mentary organs may be utterly aborted; and tins 
implies, that we find in an animal or plant no trace of 
an organ, which analogy would lead us to expect to find, 
and winch is occasionally found in monstrous individuals 
of the species. Thus in the snapdragon (antirrhinum) 
we generally do not find a rudiment of a fifth stamen ; 
but this may sometimes be seen. In tracing the 
homologies of the same part in different members of a 
class, nothing is more common, or more necessary, than 
the use and discovery of rudiments. This is well shown 
in the drawings given by Owen of the bones of the leg 
of the horse, ox, and rhinoceros. 

It is an important fact that rudimentary organs, such 
as teeth in the upper jaws of whales and ruminants, 
can often be detected in the embryo, but afterwards 
wholly disappear. It is also, I believe, a universal 


rule, that a rudimentary part or organ is of greater 
size relatively to the adjoining parts in the embryo, 
than in the adult ; so that the organ at this early age 
is less rudimentary, or even cannot be said to be in any 
degree rudimentary. Hence, also, a rudimentary organ 
in the adult, is often said to have retained its embryonic 

I have now given the leading facts with respect to 
rudimentary organs. In reflecting on them, every one 
must be struck with astonishment : for the same reason- 
ing power which tells us plainly that most parts and 
organs are exquisitely adapted for certain purposes, 
tells us with equal plainness that these rudimentary or 
atrophied organs, are imperfect and useless. In works 
on natural history rudimentary organs are gene- 
rally said to have been created " for the sake of sym- 
metry," or in order " to complete the scheme of nature ;" 
but this seems to me no explanation, merely a re- 
statement of the fact. Would it be thought sufficient : 
to say that because planets revolve in elliptic courses 
round the sun, satellites follow the same course round the 
planets, for the sake of symmetry, and to complete the 
scheme of nature? An eminent physiologist accounts 
for the presence of rudimentary organs, by supposing 
that they serve to excrete matter in excess, or injurious 
to the system ; but can we suppose that the minute pa- 
pilla, which often represents the pistil in male flowers, 
/"and which is formed merely of cellular tissue} can thus 
act ? Can we suppose that the formation of rudimentary 
teeth which are subsequently absorbed, can be of any ( 
service to the rapidly growing embryonic calf by the 
excretion of precious phosphate of lime ? When a man's ) 
fingers have been amputated, imperfect nails sometimes 
appear on the stumps : I could as soon believe that these 
vestiges of nails have appeared, not from unknown laws 


of growth, but in order to excrete horny matter, as that 
the rudimentary nails on the fin of the manatee were 
formed for this purpose. 

On my view of descent with modification, the origin 
of rudimentary organs is simple. We have plenty of 
cases of rudimentary organs in our domestic produc- 
tions, — as the stump of a tail in tailless breeds, — the 
vestige of an ear in earless breeds, — the reappearance 
of minute dangling horns in hornless breeds of cattle, 
more especially, according to Youatt, in young animals, 
— and the state of the whole flower in the cauliflower. 
We often see rudiments of various parts in monsters. 
But I doubt whether any of these cases throw light on 
the origin of rudimentary organs in a state of nature, 
further than by showing that rudiments can be pro- 
duced ; for I doubt whether species under nature ever 
undergo abrupt changes. I believe that disuse has been 
the main agency ; that it has led in successive genera- 
tions to the gradual reduction of various organs, until 
they have become rudimentary, — as in the case of the 
eyes of animals inhabiting dark caverns, and of the 
wings of birds inhabiting oceanic islands, which have 
seldom been forced to take flight, and have ultimately 
lost the power of flying. Again, an organ useful under 
certain conditions, might become injurious under others, 
as with the wings of beetles living on small and exposed 
islands ; and in this case natural selection would con- 
tinue slowly to reduce the organ, until it was rendered 
harmless and rudimentary. 

Any change in function, which can be effected by 
insensibly small steps, is within the power of natural 
selection ; so that an organ rendered, during changed 
habits of life, useless or injurious for one purpose, might 
easily be modified and used for another purpose. Or 
an organ might be retained for one alone of its 


former functions. An organ, when rendered useless, 
may well be variable, for its variations cannot be 
checked by natural selection. At whatever period of 
life disuse or selection reduces an organ, and this will 
generally be when the being has come to maturity and 
to its full powers of action, the principle of inheritance 
at corresponding ages will reproduce the organ in its 
reduced state at the same age, and consequently will 
seldom affect or reduce it in the embryo. Thus we can 
understand the greater relative size of rudimentary 
organs in the embryo, and their lesser relative size in 
the adult. But if each step of the process of reduction 
were to be inherited, not at the corresponding age, but 
at an extremely early period of life (as we have good 
reason to believe to be possible) the rudimentary part 
would tend to be wholly lost, and we should have a case 
of complete abortion. The principle, also, of economy, 
explained in a former chapter, by which the materials 
forming any part or structure, if not useful to the pos- 
sessor, will be saved as far as is possible, will probably 
often come into play ; and this will tend to cause the 
entire obliteration of a rudimentary organ. 

As the presence of rudimentary organs is thus 
due to the tendency in every part of the organisation, 
which has long existed, to be inherited — we can under- 
stand, on the genealogical view of classification, how it is 
that systematists have found rudimentary parts as useful 
as, or even sometimes more useful than, parts of high 
physiological importance. Eudimentary organs may be 
compared with the letters in a word, still retained in 
the spelling, but become useless in the pronunciation, 
but which serve as a clue in seeking for its derivation. 
On the view of descent with modification, we may con- 
clude that the existence of organs in a rudimentary, 
imperfect, and useless condition, or quite aborted, far 

456 SUMMARY. Chap. XIII. 

from presenting a strange difficulty, as they assuredly 
do on the ordinary doctrine of creation, might even 
have been anticipated, and can be accounted for by the 
laws of inheritance. 

Summary. — In this chapter I have attempted to show, 
that the subordination of group to group in all organisms 
throughout all time ; that the nature of the relationship, 
by which all living and extinct beings are united by 
complex, radiating, and circuitous lines of affinities into 
one grand system ; the rules followed and the difficulties 
encountered by naturalists in their classifications ; the 
value set upon characters, if constant and prevalent, 
whether of high vital importance, or of the most trifling 
importance, or, as in rudimentary organs, of no import- 
ance ; the wide opposition in value between analogical 
or adaptive characters, and characters of true affinity ; 
and other such rules ; — all naturally follow on the view 
of the common parentage of those forms which are con- 
sidered by naturalists as allied, together with their modi- 
fication through natural selection, with its contingencies 
of extinction and divergence of character. In consider- 
ing tins view of classification, it should be borne in 
mind that the element of descent has been universally 
used in ranking together the sexes, ages, and acknow- 
ledged varieties of the same species, however different 
they may be in structure. If we extend the use of 
this element of descent, — the only certainly known 
cause of similarity in organic beings, — we shall under- 
stand what , is meant by the natural system : it is 
genealogical in its attempted arrangement, with the 
grades of acquired difference marked by the terms 
varieties, species, genera, families, orders, and classes. 

On this same view of descent with modification, all 
the great facts in Morphology become intelligible, — 

Chap. XIII. SUMMARY. 457 

whether we look to the same pattern displayed in the 
homologous organs, to whatever purpose applied, of the 
different species of a class ; or to the homologous parts 
constructed on the same pattern in each individual 
animal and plant. 

On the principle of successive slight variations, not 
necessarily or generally supervening at a very early 
period of life, and being inherited at a corresponding 
period, we can understand the great leading facts in 
Embryology ; namely, the resemblance in an indivi- 
dual embryo of the homologous parts, which when ma- 
tured will become widely different from each other 
in structure and function; and the resemblance in 
different species of a class of the homologous parts or 
organs, though fitted in the adult members for pur- 
poses as different as possible. Larvae are active em- 
bryos, which have become specially modified in relation 
to their habits of life, through the principle of modifica- 
tions being inherited at corresponding ages. On this 
same principle — and bearing in mind, that when organs 
are reduced in size, either from disuse or selection, it 
will generally be at that period of life when the being 
has to provide for its own wants, and bearing in mind 
how strong is the principle of inheritance — the occur- 
rence of rudimentary organs and their final abortion, 
present to us no inexplicable difficulties ; on the con- 
trary, their presence might have been even anticipated. 
The importance of embryological characters and of 
rudimentary organs in classification is intelligible, on 
the view that an arrangement is only so far natural as 
it is genealogical. 

Finally, the several classes of facts which have been 
considered in this chapter, seem to me to proclaim 
so plainly, that the inumerable species, genera, and 
families of organic beings, with which this world is 

458 SUMMARY. Chap. XIII. 

peopled, have all descended, each within its own class 
or group, from common parents, and have all been 
modified in the course of descent, that I should without 
hesitation adopt this view, even if it were unsupported 
by other facts or arguments. 



Recapitulation and Conclusion. 

Recapitulation of the difficulties on the theory of Natural Selection 
— Recapitulation of the general and special circumstances in its 
favour — Causes of the general belief in the immutability of 
species — How far the theory of natural selection may be 
extended — Effects of its adoption on the study of Natural 
history — Concluding remarks. 

As this whole volume is one long argument, it may be 
convenient to the reader to have the leading facts and 
inferences briefly recapitulated. 

That many and grave objections may be advanced 
against the theory of descent with modification through 
natural selection, I do not deny. I have endeavoured 
to give to them their full force. Nothing at first can 
appear more difficult to believe than that the more 
complex organs and instincts should have been per- 
fected, not by means superior to, though analogous 
with, human reason, but by the accumulation of innumer- 
able slight variations, each good for the individual pos- 
sessor. Nevertheless, this difficulty, though appearing 
to our imagination insuperably great, cannot be con- 
sidered real if we admit the following propositions, 
namely, — that gradations in the perfection of any organ 
or instinct, which we may consider, either do now exist 
or could have existed, each good of its kind, — that all 
organs and instincts are, in ever so slight a degree, 
variable, — and, lastly, that there is a struggle for exist- 
ence leading to the preservation of each profitable 
deviation of structure or instinct. The truth of these 
propositions cannot, I think, be disputed. 



It is, no doubt, extremely difficult even to conjecture 
by what gradations many structures have been per- 
fected, more especially amongst broken and failing 
groups of organic beings ; but we see so many strange 
gradations in nature, as is proclaimed by the canon, 
" Natura non facit saltum," that we ought to be ex- 
tremely cautious in saying that any organ or instinct, 
or any whole being, could not have arrived at its present 
state by many graduated steps. There are, it must be 
admitted, cases of special difficulty on the theory of 
natural selection ; and one of the most curious of these 
is the existence of two or three defined castes of workers 
or sterile females in the same community of ants ; but 
I have attempted to show how this difficulty can be 

With respect to the almost universal sterility of 
species when first crossed, which forms so remarkable 
a contrast with the almost universal fertility of varieties 
when crossed, I must refer the reader to the recapitula- 
tion of the facts given at the end of the eighth chapter, 
which seem to me conclusively to show that this sterility 
is no more a special endowment than is the incapacity 
of two trees to be grafted together ; but that it is inci- 
dental on constitutional differences in the reproductive 
systems of the intercrossed species. We see the truth 
of this conclusion in the vast difference in the result, 
when the same two species are crossed reciprocally ; 
that is, when one species is first used as the father and 
then as the mother. 

The fertility of varieties when intercrossed and of 
their mongrel offspring cannot be considered as uni- 
versal ; nor is their very general fertility surprising 
when we remember that it is not likely that either 
their constitutions or their reproductive systems should 
have been profoundly modified. Moreover, most of the 


varieties which have been experimentised on have been 
produced under domestication ; and as domestication 
apparently tends to eliminate sterility, we ought not to 
expect it also to produce sterility. 

The sterility of hybrids is a very different case from 
that of first crosses, for their reproductive organs are 
more or less functionally impotent; whereas in first 
crosses the organs on both sides are in a perfect con- 
dition. As we continually see that organisms of all 
kinds are rendered in some degree sterile from their 
constitutions having been disturbed by slightly dif- 
ferent and new conditions of life, we need not feel 
surprise at hybrids being in some degree sterile, for 
their constitutions can hardly fail to have been dis- 
turbed from being compounded of two distinct organisa- 
tions. This parallelism is supported by another parallel, 
but directly opposite, class of facts ; namely, that the 
vigour and fertility of all organic beings are increased 
by slight changes in their conditions of life, and that 
the offspring of slightly modified forms or varieties ac- 
quire from being crossed increased vigour and fertility. 
So that, on the one hand, considerable changes in the 
conditions of life and crosses between greatly modified 
forms, lessen fertility ; and on the other hand, lesser 
changes in the conditions of life and crosses between 
less modified forms, increase fertility. 

Turning to geographical distribution, the difficulties 
encountered on the theory of descent with modification 
are grave enough. All the individuals of the same 
species, and all the species of the same genus, or even 
higher group, must have descended from common 
parents ; and therefore, in however distant and isolated 
parts of the world they are now found, they must in the 
course of successive generations have passed from some 
one part to the others. We are often wholly unable 


even to conjecture how this could have been effected. 
Yet, as we have reason to believe that some species have 
retained the same specific form for very long periods, 
enormously long as measured by years, too much stress 
ought not to be laid on the occasional wide diffusion of 
the same species ; for during very long periods of time 
there will always be a good chance for wide migra- 
tion by many means. A broken or interrupted range 
may often be accounted for by the extinction of the 
species in the intermediate regions. It cannot be de- 
nied that we are as yet very ignorant of the full extent 
of the various climatal and geographical changes which 
have affected the earth during modern periods ; and 
such changes will obviously have greatly facilitated mi- 
gration. As an example, I have attempted to show 
how potent has been the influence of the Glacial period 
on the distribution both of the same and of representa- 
tive species throughout the world. We are as yet pro- 
foundly ignorant of the many occasional means of trans- 
port. With respect to distinct species of the same 
genus inhabiting very distant and isolated regions, as 
the process of modification has necessarily been slow, all 
the means of migration will have been possible during a 
very long period ; and consequently the difficulty of the 
wide diffusion of species of the same genus is in some 
degree lessened. 

As on the theory of natural selection an interminable 
number of intermediate forms must have existed, linking 
together all the species in each group by gradations as 
fine as our present varieties, it may be asked, Why do 
we not see these linking forms all around us ? Why 
are not all organic beings blended together in an inex- 
tricable chaos ? With respect to existing forms, we 
should remember that we have no right to expect (ex- 
cepting in rare cases) to discover directly connecting 


links between them, but only between each and some 
extinct and supplanted form. Even on a wide area, 
which has during a long period remained continuous, 
and of which the climate and other conditions of life 
change insensibly in going from a district occupied by 
one species into another district occupied by a closely 
allied species, we have no just right to expect often to 
find intermediate varieties in the intermediate zone. 
For we have reason to believe that only a few species 
are undergoing change at any one period ; and all 
changes are slowly effected. I have also shown that the 
intermediate varieties which will at first probably exist 
in the intermediate zones, will be liable to be sup- 
planted by the allied forms on either hand ; and the 
latter, from existing in greater numbers, will generally 
be modified and improved at a quicker rate than the 
intermediate varieties, which exist in lesser numbers ; so 
that the intermediate varieties will, in the long run, be 
supplanted and exterminated. 

On this doctrine of the extermination of an infinitude 
of connecting links, between the living and extinct in- 
habitants of the world, and at each successive period 
between the extinct and still older species, why is not 
every geological formation charged with such links ? 
Why does not every collection of fossil remains afford 
plain evidence of the gradation and mutation of the 
forms of life ? We meet with no such evidence, and tins 
is the most obvious and forcible of the many objections 
winch may be urged against my theory. Why, again, 
do whole groups of allied species appear, though cer- 
tainly they often falsely appear, to have come in sud- 
denly on the several geological stages ? Why do we not 
find great piles of strata beneath the Silurian system, 
stored with the remains of the progenitors of the Silurian 
groups of fossils ? For certainly on my theory such 


strata must somewhere have been deposited at these 
ancient and utterly unknown epochs in the world's 

I can answer these questions and grave objections 
only on the supposition that the geological record is far 
more imperfect than most geologists believe. It cannot 
be objected that there has not been time sufficient for 
any amount of organic change ; for the lapse of time 
has been so great as to be utterly inappreciable by the 
human intellect. The number of specimens in all our 
museums is absolutely as nothing compared with the 
countless generations of countless species which certainly 
have existed. We should not be able to recognise a 
species as the parent of any one or more species if we 
were to examine them ever so closely, unless we like- 
wise possessed many of the intermediate links between 
their past or parent and present states ; and these many 
links we could hardly ever expect to discover, owing to 
the imperfection of the geological record. Numerous 
existing doubtful forms could be named which are pro- 
bably varieties ; but who will pretend that in future 
ages so many fossil links will be discovered, that natu- 
ralists will be able to decide, on the common view, 
whether or not these doubtful forms are varieties ? As 
long as most of the links between any two species are 
unknown, if any one link or intermediate variety be dis- 
covered, it will simply be classed as another and distinct 
species. Only a small portion of the world has been 
geologically explored. Only organic beings of certain 
classes can be preserved in a fossil condition, at least 
in any great number. Widely ranging species vary 
most, and varieties are often at first local, — both causes 
rendering the discovery of intermediate links less likely. 
Local varieties will not spread into other and distant 
regions until they are considerably modified and im- 


proved ; and when they do spread, if discovered in a 
geological formation, they will appear as if suddenly 
created there, and will be simply classed as new species. 
Most formations have been intermittent in their ac- 
cumulation ; and their duration, I am inclined to believe, 
has been shorter than the average duration of specific 
forms. Successive formations are separated from each 
other by enormous blank intervals of time ; for fossili- 
ferous formations, thick enough to resist future de- 
gradation, can be accumulated only where much sedi- 
ment is deposited on the subsiding bed of the sea. 
During the alternate periods of elevation and of station- 
ary level the record will be blank. During these latter 
periods there will probably be more variability in the 
forms of life ; during periods of subsidence, more ex- 

With respect to the absence of fossiliferous formations 
beneath the lowest Silurian strata, I can only recur to 
the hypothesis given in the ninth chapter. That the 
geological record is imperfect all will admit ; but that 
it is imperfect to the degree which I require, few will 
be inclined to admit. If we look to long enough in- 
tervals of time, geology plainly declares that all species 
have changed ; and they have changed in the manner 
which my theory requires, for they have changed slowly 
and in a graduated manner. We clearly see this in 
the fossil remains from consecutive formations invariably 
being much more closely related to each other, than are 
the fossils from formations distant from each other in 

Such is the sum of the several chief objections and 
difficulties which may justly be urged against my theory ; 
and I have now briefly recapitulated the answers and 
explanations which can be given to them. I have felt 
these difficulties far too heavily during many years to 



doubt their weight. Bat it deserves especial notice 
that the more important objections relate to questions 
on which we are confessedly ignorant ; nor do we know 
how ignorant we are. We do not know all the possible 
transitional gradations between the simplest and the 
most perfect organs ; it cannot be pretended that we 
know all the varied means of Distribution during the 
long lapse of years, or that we know how imperfect the 
Geological Kecord is. Grave as these several difficulties 
are, in my judgment they do not overthrow the theory 
of descent with modification. 

Now let us turn to the other side of the argument. 
Under domestication we see much variability. This 
seems to be mainly due to the reproductive system 
being eminently susceptible to changes in the conditions 
of life ; so that this system, when not rendered impotent, 
fails to reproduce offspring exactly like the parent-form. 
Variability is governed by many complex laws, — by 
correlation of growth, by use and disuse, and by the 
direct action of the physical conditions of life. There is 
much difficulty in ascertaining how much modification 
our domestic productions have undergone ; but we may 
safely infer that the amount has been large, and that 
modifications can be inherited for long periods. As 
long as the conditions of life remain the same, we have 
reason to believe that a modification, which has already 
been inherited for many generations, may continue 
to be inherited for an almost infinite number of 
generations. On the other hand we have evidence 
that variability, when it has once come into play, 
does not wholly cease ; for new varieties are still occa- 
sionally produced by our most anciently domesticated 

Man does not actually produce variability ; he only 


unintentionally exposes organic beings to new conditions 
of life, and then nature acts on the organisation, and 
causes variability. But man can and does select the 
variations given to him by nature, and thus accumulate 
them in any desired manner. He thus adapts animals 
and plants for his own benefit or pleasure. He may do 
this methodically, or he may do it unconsciously by pre- 
serving the individuals most useful to him at the time, 
without any thought of altering the breed. It is cer- 
tain that he can largely influence the character of a 
breed by selecting, in each successive generation, indi- 
vidual differences so slight as to be quite inappreciable 
by an uneducated eye. This process of selection has 
been the great agency in the production of the most 
distinct and useful domestic breeds. That many of the 
breeds produced by man have to a large extent the cha- 
racter of natural species, is shown by the inextricable 
doubts whether very many of them are varieties or 
aboriginal species. 

There is no obvious reason why the principles which 
have acted so efficiently under domestication should not 
have acted under nature. In the preservation of favoured 
individuals and races, during the constantly-recurrent 
Struggle for Existence, we see the most powerful and 
ever-acting means of selection. The struggle for exist- 
ence inevitably follows from the high geometrical ratio 
of increase which is common to all organic beings. 
This high rate of increase is proved by calculation, by 
the effects of a succession of peculiar seasons, and by 
the results of naturalisation, as explained in the third 
chapter. More individuals are born than can possibly 
survive. A grain in the balance will determine which 
individual shall live and which shall die, — which variety 
or species shall increase in number, and which shall 
decrease, or finally become extinct. As the indi- 


victuals of the same species come in all respects into the 
closest competition with each other, the struggle will 
generally be most severe between them ; it will be almost 
equally severe between the varieties of the same species, 
and next in severity between the species of the same 
genus. But the struggle will often be very severe be- 
tween beings most remote in the scale of nature. The 
slightest advantage in one being, at any age or during 
any season, over those with which it comes into compe- 
tition, or better adaptation in however slight a degree to 
the surrounding physical conditions, will turn the balance. 

With animals having separated sexes there will in 
most cases be a struggle between the males for possession 
of the females. The most vigorous individuals, or those 
which have most successfully struggled with their condi- 
tions of life, will generally leave most progeny. But 
success will often depend on having special weapons or 
means of defence, or on the charms of the males ; and 
the slightest advantage will lead to victory. 

As geology plainly proclaims that each land has 
undergone great physical changes, we might have ex- 
pected that organic beings would have varied under 
nature, in the same way as they generally have varied 
under the changed conditions of domestication. And 
if there be any variability under nature, it would be an 
unaccountable fact if natural selection had not come 
into play. It has often been asserted, but the assertion 
is quite incapable of proof, that the amount of variation 
under nature is a strictly limited quantity. Man, 
though acting on external characters alone and often 
capriciously, can produce within a short period a great 
result by adding up mere individual differences in his 
domestic productions ; and every one admits that there 
are at least individual differences in species under 
nature. But, besides such differences, all naturalists 


have admitted the existence of varieties, which they 
think sufficiently distinct to be worthy of record in 
systematic works. No one can draw any clear distinc- 
tion between individual differences and slight varieties ; 
or between more plainly marked varieties and sub-spe- 
cies, and species. Let it be observed how naturalists 
differ in the rank which they assign to the many repre- 
sentative forms in Europe and North America. 

If then we have under nature variability and a power- 
ful agent always ready to act and select, why should we 
doubt that variations in any way useful to beifl gs, nndetf 
their excessively complex relations of life, would b> 
served, accumulated, and inherited ? Why, if man can 
by patience select variations most useful to himself, 
should nature fail in selecting variations useful, under 
changing conditions of life, to her living products? 
What limit can be put to this power, acting during long 
ages and rigidly scrutinising the whole constitution, 
structure, and habits of each creature, — favouring the 
good and rejecting the bad ? I can see no limit to this 
power, in slowly and beautifully adapting each form to 
the most complex relations of life. The theory of 
natural selection, even if we looked no further than this, 
seems to me to be in itself probable. I have already 
recapitulated, as fairly as I could, the opposed difficulties 
and objections : now let us turn to the special facts and 
arguments in favour of the theory. 

On the view that species are only strongly marked 
and permanent varieties, and that each species first 
existed as a variety, we can see why it is that no line 
of demarcation can be drawn between species, com- 
monly supposed to have been produced by special acts 
of creation, and varieties which are acknowledged to 
have been produced by secondary laws. On this same 
view we can understand how it is that in each region 


where many species of a genus have been produced, 
and where they now flourish, these same species should 
present many varieties ; for where the manufactory of 
species has been active, we might expect, as a general 
rule, to find it still in action ; and this is the case if 
varieties be incipient species. Moreover, the species of 
the larger genera, which afford the greater number of 
varieties or incipient species, retain to a certain degree 
the character of varieties ; for they differ from each 
other by a less amount of difference than do the spe- 
cies of smaller genera. The closely allied species also 
of the larger genera apparently have restricted ranges, 
and they are clustered in little groups round other spe- 
cies — in which resjDects they resemble varieties. These 
are strange relations on the view of each species having 
been independently created, but are intelligible if all 
species first existed as varieties. 

As each species tends by its geometrical ratio of 
reproduction to increase inordinately in number; and 
as the modified descendants of each species will be 
enabled to increase by so much the more as they 
become more diversified in habits and structure, so as 
to be enabled to seize on many and widely different 
places in the economy of nature, there will be a con- 
stant tendency in natural selection to preserve the most 
divergent offspring of any one species. Hence during a 
long-continued course of modification, the slight differ- 
ences, characteristic of varieties of the same species, 
tend to be augmented into the greater differences cha- 
racteristic of species of the same genus. New and im- 
proved varieties will inevitably supplant and exterminate 
the older, less improved and intermediate varieties ; and 
thus species are rendered to a large extent defined and 
distinct objects. Dominant species belonging to the 
larger groups tend to give birth to new and dominant 


forms ; so that eacli large group tends to become still 
larger, and at the same time more divergent in character. 
But as all groups cannot thus succeed in increasing in 
size, for the world would not hold them, the more domi- 
nant groups beat the less dominant. This tendency 
in the large groups to go on increasing in size and 
diverging in character, together with the almost in- 
evitable contingency of much extinction, explains the 
arrangement of all the forms of life, in groups sub- 
ordinate to groups, all within a few great classes, which 
we now see everywhere around us, and which has pre- 
vailed throughout all time. This grand fact of the 
grouping of all organic beings seems to me utterly 
inexplicable on the theory of creation. 

As natural selection acts solely by accumulating 
slight, successive, favourable variations, it can produce 
no great or sudden modification ; it can act only by very 
short and slow steps. Hence the canon of " Natura non 
facit saltum," winch every fresh addition to our know- 
ledge tends to make more strictly correct, is on this 
theory simply intelligible. We can plainly see why 
nature is prodigal in variety, though niggard in innova- 
tion. But why this should be a law of nature if each 
species has been independently created, no man can 

Many other facts are, as it seems to me, explicable 
on this theory. How strange it is that a bird, under 
the form of woodpecker, should have been created to 
prey on insects on the ground ; that upland geese, 
which never or rarely swim, should have been created 
with webbed feet ; that a thrush should have been 
created to dive and feed on sub-aquatic insects; and 
that a petrel should have been created witli habits and 
structure fitting it for the life of an auk or grebe ! and 
so on in endless other cases. But on the view of each 


species constantly trying to increase in number, with 
natural selection always ready to adapt the slowly vary- 
ing descendants of each to any unoccupied or ill-occu- 
pied place in nature, these facts cease to be strange, or 
perhaps might even have been anticipated. 

As natural selection acts by competition, it adapts 
the inhabitants of each country only in relation to the 
degree of perfection of their associates ; so that we 
need feel no surprise at the inhabitants of any one 
country, although on the ordinary view supposed to have 
been specially created and adapted for that country, 
being beaten and supplanted by the naturalised produc- 
tions from another land. Nor ought we to marvel if all 
the contrivances in nature be not, as far as we can 
judge, absolutely perfect ; and if some of them be ab- 
horrent to our ideas of fitness. We need not marvel at 
the sting of the bee causing the bee's own death ; at 
drones being produced in such vast numbers for one 
single act, and being then slaughtered by their sterile 
sisters ; at the astonishing waste of pollen by our fir- 
trees ; at the instinctive hatred of the queen bee for her 
own fertile daughters ; at ichneumonidae feeding within 
the live bodies of caterpillars ; and at other such cases. 
The wonder indeed is, on the theory of natural selection, 
that more cases of the want of absolute perfection have 
not been observed. 

The complex and little known laws governing varia- 
tion are the same, as far as we can see, with the laws 
which have governed the production of so-called specific 
forms. In both cases physical conditions seem to have 
produced but little direct effect ; yet when varieties 
enter any zone, they occasionally assume some of the 
characters of the species proper to that zone. In both 
varieties and species, use and disuse seem to have pro- 
duced some effect ; for it is difficult to resist this con- 


elusion when we look, for instance, at the logger-headed 
duck, winch has wings incapable of flight, in nearly 
the same condition as in the domestic duck ; or when 
we look at the burrowing tucutucu, which is occasionally 
blind, and then at certain moles, which are habitually 
blind and have their eyes covered with skin ; or when 
we look at the blind animals inhabiting the dark caves 
of America and Europe. In both varieties and species 
correlation of growth seems to have played a most im- 
portant part, so that when one part has been modified 
other parts are necessarily modified. In both varieties 
and species reversions to long-lost characters occur. 
How inexplicable on the theory of creation is the occa- 
sional appearance of stripes on the shoulder and legs 
of the several species of the horse-genus and in their 
hybrids! How simply is this fact explained if we 
believe that these species have descended from a striped 
progenitor, in the same manner as the several domestic 
breeds of pigeon have descended from the blue and 
barred rock-pigeon ! 

On the ordinary view of each species having been 
independently created, why should the specific charac- 
ters, or those by which the species of the same genus 
differ from each other, be more variable than the generic 
characters in which they all agree ? Why, for instance, 
should the colour of a flower be more likely to vary in 
any one species of a genus, if the other species, supposed 
to have been created independently, have differently co- 
loured flowers, than if all the species of the genus have the 
same coloured flowers ? If species are only well-marked 
varieties, of which the characters have become in a high 
degree permanent, we can understand this fact; for 
they have already varied since they branched off from a 
common progenitor in certain characters, by which they 
have come to be specifically distinct from each other ; 


and therefore these same characters would be more 
likely still to be variable than the generic characters 
which have been inherited without change for an enor- 
mous period. It is inexplicable on the theory of crea- 
tion why a part developed in a very unusual manner in 
any one species of a genus, and therefore, as we may 
naturally infer, of great importance to the species, should 
be eminently liable to variation ; but, on my view, this 
part has undergone, since the several species branched 
off from a common progenitor, an unusual amount of 
variability and modification, and therefore we might 
expect this part generally to be still variable. But a 
part may be developed in the most unusual manner, 
like the wing of a bat, and yet not be more variable 
than any other structure, if the part be common to 
many subordinate forms, that is, if it has been inherited 
for a very long period ; for in this case it will have been 
rendered constant by long-continued natural selection. 

Glancing at instincts, marvellous as some are, they 
offer no greater difficulty than does corporeal structure 
on the theory of the natural selection of successive, slight, 
but profitable modifications. We can thus understand 
why nature moves by graduated steps in endowing dif- 
ferent animals of the same class with their several in- 
stincts. I have attempted to show how much light the 
principle of gradation throws on the admirable archi- 
tectural powers of the hive-bee. Habit no doubt some- 
times comes into play in modifying instincts ; but it 
certainly is not indispensable, as we see, in the case of 
neuter insects, which leave no progeny to inherit the 
effects of long-continued habit. On the view of all the 
species of the same genus having descended from a 
common parent, and having inherited much in common, 
we can understand how it is that allied species, when 
placed under considerably different conditions of life 


yet should follow nearly the same instincts ; why the 
thrush of South America, for instance, lines her nest 
with mud like our British species. On the view of 
instincts having been slowly acquired through natural 
selection we need not marvel at some instincts being 
apparently not perfect and liable to mistakes, and at 
many instincts causing other animals to suffer. 

If species be only well-marked and permanent varie- 
ties, we can at once see why their crossed offspring 
should follow the same complex laws in their degrees 
and kinds of resemblance to their parents, — in being ab- 
sorbed into each other by successive crosses, and in other 
such points, — as do the crossed offspring of acknow- 
ledged varieties. On the other hand, these would be 
strange facts if species have been independently created, 
and varieties have been produced by secondary laws. 

If we admit that the geological record is imperfect 
in an extreme degree, then such facts as the record 
gives, support the theory of descent with modification. 
New species have come on the stage slowly and at 
successive intervals ; and the amount of change, after 
equal intervals of time, is widely different in different 
groups. The extinction of species and of whole groups 
of species, which has played so conspicuous a part in the 
history of the organic world, almost inevitably follows 
on the principle of natural selection ; for old forms will 
be supplanted by new and improved forms. Neither 
single species nor groups of species reappear when the 
chain of ordinary generation has once been broken. 
The gradual diffusion of dominant forms, with the slow- 
modification of their descendants, causes the forms of 
life, after long intervals of time, to appear as if they 
had changed simultaneously throughout the world. The 
fact of the fossil remains of each formation being in 
some degree intermediate in character between the 


fossils in the formations above and below, is simply 
explained by their intermediate position in the chain of 
descent. The grand fact that all extinct organic beings 
belong to the same system with recent beings, falling 
either into the same or into intermediate groups, follows 
from the living and the extinct being the offspring 
of common parents. As the groups which have de- 
scended from an ancient progenitor have generally 
diverged in character, the progenitor with its early de- 
scendants will often be intermediate in character in 
comparison with its later descendants ; and thus we can 
see why the more ancient a fossil is, the oftener it stands 
in some degree intermediate between existing and allied 
groups. Kecent forms are generally looked at as* being, 
in some vague sense, higher than ancient and extinct 
forms ; and they are in so far higher as the later and 
more improved forms have conquered the older and less 
improved organic beings in the struggle for life. Lastly, 
the law of the long endurance of allied forms on the 
same continent, — of marsupials in Australia, of edentata 
in America, and other such cases, — is intelligible, for 
within a confined country, the recent and the extinct 
will naturally be allied by descent. 

Looking to geographical distribution, if we admit that 
there has been during the long course of ages much 
migration from one part of the world to another, owing 
to former climatal and geographical changes and to 
the many occasional and unknown means of dispersal, 
then we can understand, on the theory of descent with 
modification, most of the great leading facts in Distribu- 
tion. We can see why there should be so striking a 
parallelism in the distribution of organic beings through- 
out space, and in their geological succession throughout 
time ; for in both cases the beings have been connected 
by the bond of ordinary generation, and the means of 


modification have been the same. We see the full 
meaning of the wonderful fact, which must have struck 
every traveller, namely, that on the same continent, 
under the most diverse conditions, under heat and cold, 
on mountain and lowland, on deserts and marshes, most 
of the inhabitants within each great class are plainly 
related ; for they will generally be descendants of the 
same progenitors and early colonists. On this same 
principle of former migration, combined in most cases 
with modification, we can understand, by the aid of the 
Glacial period, the identity of some few plants, and the 
close alliance of many others, on the most distant moun- 
tains, under the most different climates ; and likewise 
the close alliance of some of the inhabitants of the sea 
in the northern and southern temperate zones, though 
separated by the whole intertropical ocean. Although 
two areas may present the same physical conditions of 
life, we need feel no surprise at their inhabitants being 
widely different, if they have been for a long period 
completely separated from each other ; for as the rela- 
tion of organism to organism is the most important of 
all relations, and as the two areas will have received 
colonists from some third source or from each other, at 
various periods and in different proportions, the course 
of modification in the two areas will inevitably be 

On this view of migration, with subsequent modifica- 
tion, we can see why oceanic islands should be inhabited 
by few species, but of these, that many should be 
peculiar. We can clearly see why those animals which 
cannot cross wide spaces of ocean, as frogs and terrestrial 
mammals, should not inhabit oceanic islands ; and why, 
on the other hand, new and peculiar species of bats, 
which can traverse the ocean, should so often be found 
on islands far distant from any continent. Such facts 


as the presence of peculiar species of bats, and the ab- 
sence of all other mammals, on oceanic islands, are 
utterly inexplicable on the theory of independent acts 
of creation. 

The existence of closely allied or representative spe- 
cies in any two areas, implies, on the theory of descent 
with modification, that the same parents formerly in- 
habited both areas ; and we almost invariably find 
that wherever many closely allied species inhabit two 
areas, some identical species common to both still exist. 
Wherever many closely allied yet distinct species occur, 
many doubtful forms and varieties of the same species 
likewise occur. It is a rule of high generality that the 
inhabitants of each area are related to the inhabitants 
of the nearest source whence immigrants might have 
been derived. We see this in nearly all the plants and 
animals of the Galapagos archipelago, of Juan Fernandez, 
and of the other American islands being related in the 
most striking manner to the plants and animals of the 
neighbouring American mainland ; and those of the 
Cape de Verde archipelago and other African islands to 
the African mainland. It must be admitted that these 
facts receive no explanation on the theory of creation. 

The fact, as we have seen, that all past and present 
organic beings constitute one grand natural system, with 
group subordinate to group, and with extinct groups 
often falling in between recent groups, is intelligible 
on the theory of natural selection with its contingencies 
of extinction and divergence of character. On these 
same principles we see how it is, that the mutual 
affinities of the species and genera within each class 
are so complex and circuitous. We see why certain 
characters are far more serviceable than others for 
classification ; — why adaptive characters, though of 
paramount importance to the being, are of hardly any 


importance in classification ; why characters derived 
from rudimentary parts, though of no service to the 
being, are often of high classificatory value ; and why 
embryological characters are the most valuable of all. 
The real affinities of all organic beings are due to inhe- 
ritance or community of descent. The natural system 
is a genealogical arrangement, in which we have to 
discover the lines of descent by the most permanent 
characters, however slight their vital importance 
may be. 

The framework of bones being the same in the hand 
of a man, wing of a bat, fin of the porpoise, and leg of 
the horse, — the same number of vertebrae forming the 
neck of the giraffe and of the elephant, — and innu- 
merable other such facts, at once explain themselves on 
the theory of descent with slow and slight successive 
modifications. The similarity of pattern in the wing and 
leg of a bat, though used for such different purpose, — in 
the jaws and legs of a crab, — in the petals, stamens, and 
pistils of a flower, is likewise intelligible on the view of 
the gradual modification of parts or organs, which were 
alike in the early progenitor of each class. On the 
principle of successive variations not always supervening 
at an early age, and being inherited at a corresponding 
not early period of life, we can clearly see why the em- 
bryos of mammals, birds, reptiles, and fishes should be 
so closely alike, and should be so unlike the adult forms. 
We may cease marvelling at the embryo of an air- 
breathing mammal or bird having branchial slits and 
arteries running in loops, like those in a fish which has 
to breathe the air dissolved in water, by the aid of well- 
developed branchia). 

Disuse, aided sometimes by natural selection, will 
often tend to reduce an organ, when it lias become 
useless by changed habits or under changed conditions 


of life ; and we can clearly understand on this view the 
meaning of rudimentary organs. But disuse and selec- 
tion will generally act on each creature, when it has 
come to maturity and has to play its full part in the 
struggle for existence, and will thus have little power 
of acting on an organ during early life ; hence the organ 
will not be much reduced or rendered rudimentary at 
this early age. The calf, for instance, has inherited 
teeth, which never cut through the gums of the upper 
jaw, from an early progenitor having well-developed 
teeth ; and we may believe, that the teeth in the 
mature animal were reduced, during successive gene- 
rations, by disuse or by the tongue and palate having 
been fitted by natural selection to browse without their 
aid ; whereas in the calf, the teeth have been left un- 
touched by selection or disuse, and on the principle of 
inheritance at corresponding ages have been inherited 
from a remote period to the present day. On the view 
of each organic being and each separate organ having 
been specially created, how utterly inexplicable it is that 
parts, like the teeth in the embryonic calf or like the 
shrivelled wings under the soldered wing-covers of some 
beetles, should thus so frequently bear the plain stamp 
of inutility ! Nature may be said to have taken pains 
to reveal, by rudimentary organs and by homologous 
structures, her scheme of modification, which it seems 
that we wilfully will not understand. 

I have now recapitulated the chief facts and consider- 
ations which have thoroughly convinced me that species 
have changed, and are still slowly changing by the pre- 
servation and accumulation of successive slight favour- 
able variations. Why, it may be asked, have all the 
most eminent living naturalists and geologists rejected 
this view of the mutability of species? It cannot be 


asserted that organic beings in a state of nature are 
subject to no variation ; it cannot be proved that the 
amount of variation in the course of long ages is a 
limited quantity ; no clear distinction has been, or can 
be, drawn between species and well-marked varieties. 
It cannot be maintained that species when intercrossed 
are invariably sterile, and varieties invariably fertile ; 
or that sterility is a special endowment and sign of 
creation. The belief that species were immutable pro- 
ductions was almost unavoidable as long as the history 
of the world was thought to be of short duration ; and 
now that we have acquired some idea of the lapse of 
time, we are too apt to assume, without proof, that the 
geological record is so perfect that it would have 
afforded us plain evidence of the mutation of species^ 
if they had undergone mutation. 

But the chief cause of our natural unwillingness to 
admit that one species has given birth to other and 
distinct species, is that we are always slow in admitting 
any great change of which we do not see the interme- 
diate steps. The difficulty is the same as that felt by 
so many geologists, when Lyell first insisted that long 
lines of inland cliffs had been formed, and great valleys 
excavated, by the slow action of the coast-waves. The 
mind cannot possibly grasp the full meaning of the 
term of a hundred million years; it cannot add up 
and perceive the full effects of many slight variations, 
accumulated during an almost infinite number of ge- 

Although I am fully convinced of the truth of the 
views given in this volume under the form of an 
abstract, I by no means expect to convince experienced 
naturalists whose minds are stocked with a multitude 
of facts all viewed, during a long course of years, from 
a point of view directly opposite to mine. It is so easy 



to hide our ignorance under such expressions as the 
" plan of creation," " unity of design," &c, and to think 
that we give an explanation when we only restate a fact. 
Any one whose disposition leads him to attach more 
weight to unexplained difficulties than to the explana- 
tion of a certain number of facts will certainly reject 
my theory. A few naturalists, endowed with much 
flexibility of mind, and who have already begun to 
doubt on the immutability of species, may be influenced 
by this volume ; but I look with confidence to the future, 
to young and rising naturalists, who will be able to 
view both sides of the question with impartiality. Who- 
ever is led to believe that species are mutable will do 
good service by conscientiously expressing his convic- 
tion ; for only thus can the load of prejudice by which 
this subject is overwhelmed be removed. 

Several eminent naturalists have of late published 
their belief that a multitude of reputed species in each 
genus are not real species ; but that other species are 
real, that is, have been independently created. This 
seems to me a strange conclusion to arrive at. They 
admit that a multitude of forms, which till lately 
they themselves thought were special creations, and 
which are still thus looked at by the majority of natu- 
ralists, and which consequently have every external 
characteristic feature of true species, — they admit that 
these have been produced by variation, but they refuse 
to extend the same view to other and very slightly 
different forms. Nevertheless they do not pretend that 
they can define, or even conjecture, which are the 
created forms of life, and which are those produced by 
secondary laws. They admit variation as a vera causa 
in one case, they arbitrarily reject it in another, without 
assigning any distinction in the two cases. The day will* 
come when this will be given as a curious illustration of 


the blindness of preconceived opinion. These authors 
seem no more startled at a miraculous act of creation 
than at an ordinary birth. But do they really believe 
that at innumerable periods in the earth's history cer- 
tain elemental atoms have been commanded suddenly 
to flash into living tissues? Do they believe that at 
each supposed act of creation one individual or many 
were produced? Were all the infinitely numerous 
kinds of animals and plants created as eggs or seed, 
or as full grown ? and in the case of mammals, were 
they created bearing the false marks of nourishment 
from the mother's womb ? Although naturalists very 
properly demand a full explanation of every difficulty 
from those who believe in the mutability of species, on 
their own side they ignore the whole subject of the first 
appearance of species in what they consider reverent 

It may be asked how far I extend the doctrine of the 
modification of species. The question is difficult to 
answer, because the more distinct the forms are which 
we may consider, by so much the arguments fall away 
in force. But some arguments of the greatest weight 
extend very far. All the members of whole classes can 
be connected together by chains of affinities, and all 
can be classified on the same principle, in groups sub- 
ordinate to groups. Fossil remains sometimes tend to 
fill up very wide intervals between existing orders. 
Organs in a rudimentary condition plainly show that an 
early progenitor had the organ in a fully developed 
state ; and this in some instances necessarily implies an 
enormous amount of modification in the descendants. 
Throughout whole classes various structures are formed 
on the same pattern, and at an embryonic age the 
species closely resemble each other. Therefore I can- 
not doubt that the theory of descent with modification 



embraces all the members of the same class. I believe 
that animals have descended from at most only four 
or five progenitors, and plants from an equal or lesser 

/'"Analogy would lead me one step further, namely, to 
the belief that all animals and plants have descended 
from some one prototype. But analogy may be a de- 
ceitful guide. Nevertheless all living things have much 
in common, in their chemical composition, their germinal 
vesicles, their cellular structure, and their laws of growth 
and reproduction. We see this even in so trifling a cir- 
cumstance as that the same poison often similarly affects 
plants and animals ; or that the poison secreted by the 
gall-fly produces monstrous growths on the wild rose or 
oak-tree. Therefore I should infer from analogy that 
probably all the organic beings which have ever lived 
on this earth have descended from some one primordial 
form, into which life was first breathed. 

When the views entertained in this volume on the 
origin of species, or when analogous views are generally 
admitted, we can dimly foresee that there will be a con- 
siderable revolution in natural history. Systematists 
will be able to pursue their labours as at present ; but 
they will not be incessantly haunted by the shadowy 
doubt whether this or that form be in essence a species. 
This I feel sure, and I speak after experience, will be 
no slight relief. The endless disputes whether or not 
some fifty species of British brambles are true species 
will cease. Systematists will have only to decide (not 
that this will be easy) whether any form be sufficiently 
constant and distinct from other forms, to be capable 
of definition ; and if definable, whether the differences 
be sufficiently important to deserve a specific name. 
This latter point will become a far more essential con- 


sideration than it is at present; for differences, how- 
ever slight, between any two forms, if not blended by 
intermediate gradations, are looked at by most natural- 
ists as sufficient to raise both forms to the rank of 
species. Hereafter we shall be compelled to acknow- 
ledge that the only distinction between species and 
well-marked varieties is, that the latter are known, or 
believed, to be connected at the present day by inter- 
mediate gradations, whereas species were formerly thus 
connected. Hence, without quite rejecting the con- 
sideration of the present existence of intermediate gra- 
dations between any two forms, we shall be led to weigh 
more carefully and to value higher the "actual amount 
of difference between them. It is quite possible that 
forms now generally acknowledged to be merely varie- 
ties may hereafter be thought worthy of specific names, 
as with the primrose and cowslip ; and in this case 
scientific and common language will come into accord- 
ance. In short, we shall have to treat species in the 
same manner as those naturalists treat genera, who 
admit that genera are merely artificial combinations 
made for convenience. This may not be a cheering 
prospect ; but we shall at least be freed from the vain 
search for the undiscovered and undiscoverable essence 
of the term species. 

The other and more general departments of natural 
history will rise greatly in interest. The terms used by 
naturalists of affinity, relationship, community of type, 
paternity, morphology, adaptive characters, rudimentary 
and aborted organs, &c, will cease to be metaphorical, 
and will have a plain signification. When we no longer 
look at an organic being as a savage looks at a ship, as at 
something wholly beyond his comprehension ; when we 
regard every production of nature as one which has had ) 
a history ; when we contemplate every complex structure 


and instinct as the summing up of many contrivances, 
each useful to the possessor, nearly in the same way as 
when we look at any great mechanical invention as the 
summing up of the labour, the experience, the reason, 
and even the blunders of numerous workmen ; when we 
thus view each organic being, how far more interesting, 
I speak from experience, will the study of natural 
history become ! 

A grand and almost untrodden field of inquiry will 
be opened, on the causes and laws of variation, on corre- 
lation of growth, on the effects of use and disuse, on 
the direct action of external conditions, and so forth. 
The study of domestic productions will rise immensely 
in value. A new variety raised by man will be a far 
more important and interesting subject for study than 
one more species added to the infinitude of already 
recorded species. Our classifications will come to be, as 
far as they can be so made, genealogies ; and will then 
truly give what may be called the plan of creation. 
The rules for classifying will no doubt become simpler 
when we have a definite object in view. We possess no 
pedigrees or armorial bearings ; and we have to dis- 
cover and trace the many diverging lines of descent in 
our natural genealogies, by characters of any kind which 
have long been inherited. Eudimentary organs will 
speak infallibly with respect to the nature of long-lost 
structures. Species and groups of species, which are 
called aberrant, and which may fancifully be called 
living fossils, will aid us in forming a picture of the 
ancient forms of life. Embryology will reveal to us the 
structure, in some degree obscured, of the prototypes of 
each great class. 

When we can feel assured that all the individuals of 
the same species, and all the closely allied species of 
most genera, have within a not very remote period de- 


scended from one parent, and have migrated from some 
one birthplace ; and when we better know the many 
means of migration, then, by the light which geology 
now throws, and will continue to throw, on former 
changes of climate and of the level of the land, we shall 
surely be enabled to trace in an admirable manner the 
former migrations of the inhabitants of the whole world. 
Even at present, by comparing the differences of the 
inhabitants of the sea on the opposite sides of a conti- 
nent, and the nature of the various inhabitants of that 
continent in relation to their apparent means of immigra- 
tion, some light can be thrown on ancient geography. 

The noble science of Geology loses glory from the 
extreme imperfection of the record. The crust of the 
earth with its embedded remains must not be looked at 
as a well-filled museum, but as a poor collection made 
at hazard and at rare intervals. The accumulation of 
each great fossiliferous formation will be recognised as 
having depended on an unusual concurrence of circum- 
stances, and the blank intervals between the successive 
stages as having been of vast duration. But we shall 
be able to gauge with some security the duration of 
these intervals by a comparison of the preceding and 
succeeding organic forms. We must be cautious in 
attempting to correlate as strictly contemporaneous 
two formations, which include few identical species, 
by the general succession of their forms of life. As 
species are produced and exterminated by slowly act- 
ing and still existing causes, and not by miraculous 
acts of creation and by catastrophes ; and as the most 
important of all causes of organic change is one which 
is almost independent of altered and perhaps suddenly 
altered physical conditions, namely, the mutual relation 
of organism to organism, — the improvement of one being 
entailing the improvement or the extermination of 


others ; it follows, that the amount of organic change in 
the fossils of consecutive formations probably serves as a 
fair measure of the lapse of actual time. A number of 
species, however, keeping in a body might remain for a 
long period unchanged, whilst within this same period, 
several of these species, by migrating into new countries 
and coming into competition with foreign associates, 
might become modified; so that we must not overrate 
the accuracy of organic change as a measure of time. 
During early periods of the earth's history, when the 
forms of life were probably fewer and simpler, the rate 
of change was probably slower ; and at the first dawn 
of life, when very few forms of the simplest structure 
existed, the rate of change may have been slow in an 
extreme degree. The whole history of the world, as at 
present known, although of a length quite incompre- 
hensible by us, will hereafter be recognised as a mere 
fragment of time, compared with the ages which have 
elapsed since the first creature, the progenitor of innu- 
merable extinct and living descendants, was created. 

In the distant future I see open fields for far more 
important researches. Psychology will be based on a 
new foundation, that of the necessary acquirement of 
each mental power and capacity by gradation. Light 
will be thrown on the origin of man and his history, 

Authors of the highest eminence seem to be fully 
satisfied with the view that each species has been inde- 
pendently created. To my mind it accords better with 
what we know of the laws impressed on matter by the 
Creator, that the production and extinction of the past 
and present inhabitants of the world should have been 
due to secondary causes, like those determining the 
birth and death of the individual. When I view all 
beings not as special creations, but as the lineal de- 
scendants of some few beings which lived long before the 


first bed of the Silurian system was deposited, they seem 
to me to become ennobled. Judging from the past, we 
may safely infer that not one living species will trans- 
mit its unaltered likeness to a distant futurity. And of 
the species now living very few will transmit progeny 
of any kind to a far distant futurity ; for the manner in 
which all organic beings are grouped, shows that the 
greater number of species of each genus, and all the 
species of many genera, have left no descendants, but 
have become utterly extinct. We can so far take a 
prophetic glance into futurity as to foretel that it will 
be the common and widely-spread species, belonging to 
the larger and dominant groups, which will ultimately 
prevail and procreate new and dominant species. As 
all the living forms of life are the lineal descendants of 
those which lived long before the Silurian epoch, we 
may feel certain that the ordinary succession by genera- 
tion has never once been broken, and that no cataclysm 
has desolated the whole world. Hence we may look 
with some confidence to a secure future of equally in- 
appreciable length. And as natural selection works 
solely by and for the good of each being, all corporeal 
and mental endowments will tend to progress towards 

It is interesting to contemplate an entangled bank, 
clothed with many plants of many kinds, with birds 
singing on the bushes, with various insects flitting about, 
and with worms crawling through the damp earth, and 
to reflect that these elaborately constructed forms, so 
different from each other, and dependent on each other 
in so complex a manner, have all been produced bj 
laws acting around us. These laws, taken in the largest 
sense, being Growth with Keprocluction ; Inheritance 
which is almost implied by reproduction ; Variability 
from the indirect and direct action of the external con- 



ditions of life, and from use and disuse ; a Katio of In- 
crease so high as to lead to a Struggle for Life, and as a 
consequence to Natural Selection, entailing Divergence 
of Character and the Extinction of less-improved forms. 
Thus, from the war of nature, from famine and death, 
the most exalted object which we are capable of con- 
ceiving, namely, the production of the higher animals, 
directly follows. There is grandeur in this view of life, 
with its several powers, having been originally breathed 
into a few forms or into one ; and that, whilst this planet 
has gone cycling on according to the fixed law of gra- 
vity, from so simple a beginning enplless , jorms most 
beautiful and most wonderful have been, and are being, 

( 491 ) 





Ants, neuter, structure of, 236. 

Aphides attended by ants, 211. 

Aberrant groups, 429. 

Aphis, development of, 442. 

Abyssinia, plants of, 375. 

Apteryx, 182. 

Acclimatisation, 139. 

Arab horses, 35. 

Affinities of extinct species, 329. 

Aralo-Caspian Sea, 339. 

of organic beings, 411. 

Archiac, M. de, on the succession of 

Agassiz on Amblyopsis, 139. 

species, 325. 

on groups of species suddenly 

Artichoke, Jerusalem, 142. 

appearing, 302, 305. 

Ascension, plants of, 389. 

on embryological succession, 338. 

Asclepias, pollen of, 193. 

on the glacial period, 366. 

Asparagus, 359. 

on embryological characters, 418. 

Aspicarpa, 417. 

on the embryos of vertebrata, 

Asses, striped, 163. 


Ateuchus, 135. 

on parallelism of embryological 

Audubon on habits of frigate-bird, 185. 

development and geological succes- 

on variation in birds'-nests, 212. 

sion, 449. 

on heron eating seeds, 387. 

Algae of New Zealand, 376. 

Australia, animals of, 116. 

Alligators, males, fighting, 88. 

, dogs of, 215. 

Amblyopsis, blind fish, 139. 

, extinct animals of, 339. 

America, North, productions allied to 

, European plants in, 375. 

those of Europe, 371. 

Azara on flies destroying cattle, 72. 

— — — , boulders and glaciers of, 

, South, no modern formations on 

Azores, flora of, 363. 


west coast, 290. 

Ammonites, sudden extinction of, 321. 

Babington, Mr., on British plants, 48. 

Anagallis, sterility of, 247. 

Balancement of growth, 147. 

Analogy of variations, 159. 

Bamboo with hooks, 197. 

Ancylus, 386. 

Barberry, flowers of, 98. 

Animals, not domesticated from being- 

Barrande, M., on Silurian colonies, 

variable, 17. 


, domestic, descended from several 

on the succession of species, 325. 

stocks, 19. 

on parallelism of palaeozoic forma- 

tions, 328. 

of Australia, 116. 

on affinities of ancient species, 

with thicker fur in cold climates, 



Barriers, importance of, 347. 

, blind, in caves, 137. 

Batrachians on islands, 393. 

, extinct, of Australia, 339. 

Bats, how structure acquired, 180. 

Anomma, 240. 

, distribution of, 394. 

Antarctic islands, ancient flora of, 399. 

Bear, catching water-insects, 184. 

Antirrhinum, 161. 

Bee, sting of, 202. 

Ants attending aphides, 211. 

, queen, killing rivals, 202. 

, slave-making instinct, 219. 

Bees fertilising flowers, 73. 




Bees, hive, not sucking the red clover, 

, , cell-making instinct, 224. 


, humble, cells of, 225. 

Cabbage, varieties of, crossed, 99. 

, parasitic, 218. 

Calceolaria, 251. 

Beetles, wingless, in Madeira, 135. 

Canary-birds, sterility of hybrids, 252. 

with deficient tarsi, 135. 

Cape de Verde islands, 398. 

Bentham, Mr., on British plants, 48. 

Cape of Good Hope, plants of, 110, 

, on classification, 419. 


Berkeley, Mr., on seeds in salt-water, 

Carrier-pigeons killed by hawks, 362. 


Cassini on flowers of composite, 145. 

Bermuda, birds of, 391. 

Catasetum, 424. 

Birds acquiring fear, 212. 

Cats, with blue eyes, deaf, 12. 

annually cross the Atlantic, 364. 

, variation in habits of, 91. 

, colour of, on continents, 132. 

curling tail when going to spring, 

, fossil, in caves of Brazil, 339. 


of Madeira, Bermuda, and Gala- 

Cattle destroying fir-trees, 71. 

pagos, 390. 

destroyed by flies in La Plata, 

, song of males, 89. 


transporting seeds, 361. 

, breeds of, locally extinct, 111. 

, waders, 386. 

, fertility of Indian and European 

, wingless, 134, 182. 

breeds, 254. 

, with traces of embryonic teeth, 

Cave, inhabitants of, blind, 137. 


Centres of creation, 352. 

Bizcacha, 349. 

Cephalopoda?, development of, 442. 

, affinities of, 429. 

Cervulus, 253. 

Bladder for swimming in fish, 190. 

Cetacea, teeth and hair, 144. 

Blindness of cave animals, 137. 

Ceylon, plants of, 375. 

Blyth, Mr., on distinctness of Indian 

Chalk formation, 322. 

cattle, 18. 

Characters, divergence of, 111. 

, on striped Hemionns, 163. 

, sexual, variable, 156. 

-, on crossed geese, 253. 

, adaptive or analogical, 427. 

Boar, shoulder-pad of, 88. 

Charlock, 76. 

Borrow, Mr., on the Spanish pointer, 

Checks to increase, 67. 


, mutual, 71. 

Bory St. Vincent on Batrachians, 

Chickens, instinctive tameness of, 216. 


Chthamalinae, 288. 

Bosquet, M., on fossil Chthamalus, 

Chthamalus, cretacean species of, 304. 


Circumstances favourable to selection of 

Boulders, erratic, on the Azores, 363. 

domestic products, 40. 

Branchiae, 190. 

to natural selection, 101. 

Brent, Mr., on house-tumblers, 214. 

Cirripedes capable of crossing, 101. 

, on hawks killing pigeons, 362. 

, carapace aborted, 148. 

Brewer, Dr., on American cuckoo, 

, their ovigerous frena, 192. 


, fossil, 304. 

Britain, mammals of, 395. 

, larva; of, 440. 

Bronu on duration of specific forms, 

Classification, 413. 


Clift, Mr., on the succession of types, 

Brown, Robert, on classification, 414. 


Buckman on variation in plants, 10. 

Climate, effects of, in checking increase 

Buzareingues on sterility of varieties, 

of beings, 68. 


, adaptation of, to organisms, 139. 





Cobites, intestine of, 190. 

De Candolle on struggle for existence, 

Cockroach, 76. 


Collections, palreontological, poor, 287. 

on umbelliferae, 146. 

Colour, influenced, by climate, 132. 

on general affinities, 430. 

, in relation to attacks by flies, 

, Alph., on low plants, widely 


dispersed, 406. 

Columba livia, parent of domestic pi- 

, , on widely-ranging plants 

geons, 23. 

being variable, 53. 

Colymbetes, 386. 

, , on naturalisation, 115. 

Compensation of growth, 147. 

, , on winged seeds, 146. 

Composite, outer and inner florets of, 

, , on Alpine species sud- 


denly becoming rare, 175. 

, male flowers of, 451. 

, , on distribution of plants 

Conclusion, general, 480. 

with large seeds, 360. 

Conditions, slight changes in, favour- 

— — , , on vegetation of Australia, 

able to fertility, 267. 


Coot, 185. 

-. , , on fresh-water plants, 386. 

Coral-islands, seeds drifted to, 360. 

, , on insular plants, 389. 

reefs, indicating movements of 

Degradation of coast-rocks, 282. 

earth, 309. 

Denudation, rate of, 285. 

Corn-crake, 185. 

of oldest rocks, 308. 

Correlation of growth in domestic pro- 

Development of ancient forms, 336. 

ductions, 11. 

Devonian system, 334. 

of growth, 143, 198. 

Dianthus, fertility of crosses, 256. 

Cowslip, 49. 

Dirt on feet of birds, 362. 

Creation, single centres of, 352. 

Dispersal, means of, 356. 

Crinum, 250. 

during glacial period, 365. 

Crosses, reciprocal, 258. 

Distribution, geographical, 346. 

Crossing of domestic animals, import- 

, means of, 356. 

ance in altering breeds, 20. 

Disuse, effects of, under nature, 134. 

, advantages of, 96. 

Divergence of character, 111. 

— — unfavourable to selection, 102. 

Division, physiological, of labour. 115. 

Crustacea of New Zealand, 376. 

Dogs, hairless, with imperfect teeth, 

Crustacean, blind, 137. 


Cryptocerus, 238. 

descended from several wild 

Ctenomys, blind, 137. 

stocks, 18. 

Cuckoo, instinct of, 216. 

, domestic instincts of, 213. 

Currants, grafts of, 262. 

, inherited civilisation of, 215. 

Currents of sea, rate of, 359. 

, fertility of breeds together, 254. 

Cuvier on conditions of existence, 

, of crosses, 268. 


, proportions of, when young, 

on fossil monkeys, 303. 


, Fred., on instinct, 208. 

Domestication, variation under, 7. 

Downing, Mr., on fruit-trees in Ame- 


rica, 85. 

Downs, North and South, 285. 

Dana, Prof., on blind cave-animals, 

Dragon-flies, intestines of, 190. 


Drift-timber, 360. 

, on relations of crustaceans of 

Driver-ant, 240. 

Japan, 372. 

Drones killed by other bees, 202. 

. on crustaceans of New Zealand, 

Duck, domestic, wings of, reduced, 11. 


, logger-headed, 182. 





Duckweed, 385. 

Fertility of hybrids, 249. 

Dugong, affinities of, 414. 

from slight changes in conditions, 

Dung-beetles with deficient tarsi, 135. 


Dyticus, 386. 

of crossed varieties, 267. 

Fir-trees destroyed by cattle, 71. 

pollen of °03 


Fish, flying, 182. 

Earl, Mr. W., on the Malay Archipe- 

, teleostean, sudden appearance 

lago, 395. 

of, 305. 

Ears, drooping, in domestic animals, 

eating seeds, 362, 387. 


, fresh-water, distribution of, 384. 

, rudimentary, 454. 

Fishes, ganoid, now confined to fresh 

Earth, seeds in roots of trees, 361. 

water, 107. 

Eciton, 238. 

, electric organs of, 192. 

Economy of organisation, 147. 

, ganoid, living in fresh water, 321. 

Edentata, teeth and hair, 144. 

of southern hemisphere, 376. 

, fossil species of, 339. 

Flight, powers of, how acquired, 182. 

Edwards, Milne, on physiological divi- 

Flowers, structure of, in relation to 

sions of labour, 115. 

crossing, 97. 

— — , on gradations of structure, 194. 

of composite and umbelliferae,144. 

, on embryological characters, 

Forbes, E., on colours of shells, 132. 


on abrupt range of shells in 

Eggs, young birds escaping from, 87. 

depth, 175. 

Electric organs, 192. 

on poorness of palseontological 

Elephant, rate of increase, 64. 

collections, 287. 

of glacial period, 141. 

• on continuous succession of 

Embryology, 439. 

genera, 316. 

Existence, struggle for, 60. 

on continental extensions, 357. 

, conditions of, 206. 

on distribution during glacial 

Extinction, as bearing on natural selec- 

period, 366. 

tion, 109. 

on parallelism in time and space, 

of domestic varieties, 111. 


, 317. 

Forests, changes in, in America, 74. 

Eye, structure of, 187. 

Formation, Devonian, 334. 

, correction for aberration, 202. 

Formations, thickness of, in Britain, 

Eyes reduced in moles, 137. 


— — , intermittent, 290. 


Formica rufescens, 219: 

sangainea, 219. 

Fabre, M., on parasitic sphex, 218. 

flava, neuter of, 239. 

Falconer, Dr., on naturalisation of 

Frena, ovigerous, of cirripedes, 192. 

plants in India, 65. 

Fresh-water productions, dispersal of, 

on fossil crocodile, 313. 


on elephants and mastodons, 334. 

Fries on species in large genera being 

and Cautley on mammals of sub- 

closelv allied to other species, 57. 

Himalayan beds, 340. 

Frigate-bird, 185. 

Falkland Island, wolf of, 393. 

Frogs on islands, 393. 

Faults, 285. 

Fruit-trees, gradual improvement of, 

Faunas, marine, 348. 


Fear instinctive in birds, 212. 

in United c tate~ 85 

Feet of birds, young molluscs adhering 

, varieties of, acclimatised in 

to, 385. 

United States, 142. 




Fuci, crossed, 258. 

Gray, Dr. Asa, on trees of United 

Fur, thicker in cold climates, 133. 

States, 100. 

Furze, 439. 

, on naturalised plants in the 


United States, 115. 

, on rarity of intermediate varie- 

Galapagos Archipelago, birds of, 390. 

ties, 176. 

, productions of, 398, 400. 

, on Alpine plants, 365. 

Galeopithecus, 181. 

, Dr. J. E., on striped mule, 165. 

Game, increase of, checked by vermin, 

Grebe, 185. 


Groups, aberrant, 429. 

Gartner on sterility of hybrids, 247, 

Grouse, colours of, 84. 


, red, a doubtful species, 49. 

on reciprocal crosses, 258. 

Growth, compensation of, 147. 

on crossed maize and verbascum, 

, correlation of, in domestic pro- 


ducts, 11. 

on comparison of hybrids and 

, correlation of, 143. 

mongrels, 272. 

Geese, fertility when crossed, 253. 


, upland, 185. 

Genealogy important in classification, 

Habit, effect of, under domestication, 



GeofFroy St. Hilaire on balancement, 

, effect of, under nature, 134. 


, diversified, of same species, 183. 

on homologous organs, 434. 

Hair and teeth, correlated, 144. 

, Isidore, on variability of re- 

Harcourt, Mr. E. V., on the birds of 

peated parts, 149. 

Madeira, 391. 

, on correlation in monstrosi- 

Hartung, M., on boulders in the Azores, 

ties, 11. 


Hazel-nuts, 359. 

Hearne on habits of bears, 184. 

Heath, changes in vegetation, 72. 

' ™ ™,.,\.k'l„ ^«J*« kJ™ ~A„„ 

monstrous, 155. 

Geographical distribution, 346. 

Heer, O., on plants of Madeira, 107. 

Geography, ancient, 487. 

Helix pomatia, 397. 

Geology, future progress of, 487. 

Helosciadium, 359. 

, imperfection of the record, 279. 

Hemionus, striped, 163. 

Giraffe, tail of, 195. 

Herbert, W., on struggle for exist- 

Glacial period, 365. 

ence, 62. 

Gmelin on distribution, 365. 

, on sterility of hybrids, 249. 

Gnathodon, fossil, 368. 

Hermaphrodites crossing, 96. 

Godwin-Austen, Mr., on the Malay 

Heron eating seed, 387. 

Archipelago, 299. 

Heron, Sir R., on peacocks, 89. 

Goethe on compensation of growth, 

Heusinger on white animals not poisoned 


by certain plants, 12. 

Gooseberry, grafts of, 262. 

Hewitt, Mr., on sterility of first crosses, 

Gould, Dr. A., on land-shells, 397. 


— — , Mr., on colours of birds, 132. 

Himalaya, glaciers of, 373. 

, on birds of the Galapagos, 398. 

, plants of, 375. 

. , on distribution of genera of birds, 

Hippeastrum, 250. 


Holly-trees, sexes of, 93. 

Gourds, crossed, 270. 

Hollyhock, varieties ofj crossed, 271. 

Grafts, capacity of, 261. 

Hooker, Dr., on trees of New Zealand, 

Grasses, varieties of, 113. 




Hooker, Dr., on acclimatisation of 
Himalayan trees, 140. 

, on flowers of umbelliferae, 145. 

■ , on glaciers of Himalaya, 373. 

, on algae of New Zealand, 376. 

, on vegetation at the base of the 

Himalaya, 378. 

, on plants of Tierra del Fuego, 

374, 378. 

, on Australian plants, 375, 399. 

i , on relations of flora of South 

America, 379. 

, on flora of the Antarctic lands, 

381, 399. 

, on the plants of the Galapagos, 

391, 398. 

Hooks on bamboos, 197. 

to seeds on islands, 392. 

Horner, Mr., on the antiquity of Egyp- 
tians, 18. 

Horns, rudimentary, 454. 

Horse, fossil, in La Plata, 318. 

Horses destroyed by flies in La Plata, 

, striped, 1G3. 

, proportions of, when young, 445. 

Horticulturists, selection applied by, 

Huber on cells of bees, 230. 

. , P., on reason blended with in- 
stinct, 208. 

, on habitual nature of instincts, 


, on slave-making ants, 219. 

, on Melipona domestica, 225. 

Humble-bees, cells of, 225. 

Hunter, J., on secondary sexual cha- 
racters, 150. 

Hutton, Captain, on crossed geese, 25 '•■ 

Huxley, Prof., on structure of herma- 
phrodites, 101. 

, on embryological succession, 338. 

, on homologous organs, 4: '>s. 

, on the development of aphis. 442. 

Hybrids and mongrels compared, 272. 

Hybridism, 245. 

Hydra, structure of, 190. 

Ibla, 148. 

Icebergs transporting seeds, 3G3. 

Increase, rate of, 63. 

Individuals, numbers favourable to se- 
lection, 102. 

, many, whether simultaneously 

created, 356. 

Inheritance, laws of, 12. 

at corresponding ages, 14, 86. 

Insects, colour of, fitted for habitations, 

, sea- side, colours of, 132. 

, blind, in caves, 138. 

, luminous, 193. 

, neuter, 236. 

Instinct, 207. 

Instincts, domestic, 213. 

Intercrossing, advantages of, 96. 

Islands, oceanic, 388. 

Isolation favourable to selection, 104. 


Japan, productions of, 372. 
Java, plants of, 375. 
Jones, Mr. J. M., on the birds of Ber- 
muda, 391. 
Jussieu on classification, 417. 

Kentucky, caves of, 137. 
Kerguelen-land, flora of, 381, 399. 
Kidney-bean, acclimatisation of, 142. 

Kidneys of birds, 144. 

Kirby on tarsi deficient in beetles, 135. 

Knight, Andrew, on cause of variation, 

Kolreuter on the barberry, 98. 

on sterility of hybrids, 247. 

on reciprocal crosses, 258. 

on crossed varieties of nicotiana, 


on crossing male and hermaphro- 
dite flowers, 451. 

Lamarck on adaptive characters, 4J7. 
Land-shells, distribution of, 397. 

of Madeira, naturalised, 402, 

Languages, classification of, 4"J'_'. 
Lapse, great, of time, 282. 
Larva;, 440. 




Laurel, nectar secreted by the leaves, 

Laws of variation, 131. 

Leech, varieties of, 76. 

Legurninosa?, nectar secreted by glands, 

Lepidosiren, 107, 330. 

Life, struggle for, 60. 

Lingula, Silurian, 306. 

Linnaeus, aphorism of, 413. 

Lion, mane of, 88. 

, young of, striped, 439. 

Lobelia fulgens, 73, 98. 

Lobelia, sterility of crosses, 250. 

Loess of the Rhine, 384. 

Lowness of structure connected with 
variability, 149. 

Lowness, related to wide distribution, 

Lubbock, Mr., on the nerves of coccus, 

Lucas, Dr. P., on inheritance, 12. 

, on resemblance of child to parent, 


Lund and Clausen on fossils of Brazil, 

Lyell, Sir C, on the struggle for exist- 
ence, 62. 

, on modern changes of the earth, 


, on measure of denudation, 283. 

, on a carboniferous land-shell, 


, on fossil whales, 303. 

, on strata beneath Silurian sys- 
tem, 307. 

, on the imperfection of the geo- 
logical record, 310. 

, on the appearance of species, 


, on Barvande's colonies, 313. 

, on tertiary formations of Europe 

and North America, 323. 

■ , on parallelism of tertiary forma- 
tions, 328. 

• , on transport of seeds by icebergs, 


, on great alternations of climate, 


, on the distribution of fresh-water 

shells, 385. 
, on land-shells of Madeira, 402. 

Lyell and Dawson on fossilized trees in 
Nova Scotia, 296. 

Macleay on analogical characters, 427. 
Madeira, plants of, 107. 

, beetles of, wingless, 135. 

, fossil land-shells of, 339. 

, birds of, 390. 

Magpie tame in Norway, 212. 
Maize, crossed, 270. 
Malay Archipelago compared with Eu- 
rope, 299. 

, mammals of, 395. 

Malpighiaceae, 417. 
Mamma?, rudimentary, 451. 
Mammals, fossil, in secondary forma- 
tion, 303. 

, insular, 393. 

Man, origin of races of, 199. 
Manatee, rudimentary nails of, 454. 
Marsupials of Australia, 116. 

, fossil species of, 339. 

Martens, M., experiment on seeds, 

Martin, Mr. W. C, on striped mules, 

Matteuchi on the electric organs of 

rays, 193. 
Matthiola, reciprocal crosses of, 258. 
Means of dispersal, 356. 
Melipona domestica, 225. 
Metamorphism of oldest rocks, 308. 
Mice destroying bees, 74. 

, acclimatisation of, 141. 

Migration, bears on first appearance ot 

fossils, 296. 
Miller, Prof., on the cells of bees, 

Mirabilis, crosses of, 258. 
Missel-thrush, 76. 
Misseltoe, complex relations of, 3.. 
.Mississippi, rate of deposition at mouth, 

Mocking-thrush of the Galapagos, 402. 
Modification of species, how far appli- 
cable, 483. 
Moles, blind, 137. 
Mongrels, fertility and sterility of, 

and hybrids compared, 272. 




Monkeys, fossil, 303. 

Monocanthus, 424. 


Mons, Van, on the origin of fruit-trees, 

29, 39. 

Oak, varieties of, 50. 

Moquin-Tandon on sea-side plants, 132. 

Onites apelles, 135. 

Morphology, 434. 

Orchis, pollen of, 193. 

Mozart, musical powers of, 209. 

Organs of extreme perfection, 186. 

Mud, seeds in, 386. 

, electric, of fishes, 192. 

Mules, striped, 165. 

of little importance, 194. 

Miiller, Dr. F., on Alpine Australian 

, homologous, 434. 

plants, 375. 

, rudiments of, 450. 

Murchison, Sir R., on the formations 

Ornithorhynchus, 107, 416. 

of Russia, 289. 

Ostrich not capable of flight, 134. 

, on azoic formations, 307. 

, habit of laying eggs together, 

, on extinction, 317. 


Mustek vison, 179. 

, American, two species of, 349. 

Myanthus, 424. 

Otter, habits of, how acquired, 179. 

Myrmecocystus, 238. 

Ouzel, water, 185. 

Myrmica, eyes of, 240. 

Owen, Prof., on birds not flying, 134. 

, on vegetative repetition, 149. 


, on variable length of arms in 

ourang-outang, 150. 

Nails, rudimentary, 453. 

, on the swim-bladder of fishes, 

Natural history, future progress of, 



, on electric organs, 192. 

selection, 80. 

, on fossil horse of La Plata, 319. 

system, 413. 

, on relations of ruminants and 

Naturalisation of forms distinct from 

pachyderms, 329. 

the indigenous species, 115. 

, on fossil birds of New Zealand, 

in New Zealand, 201. 


Nautilus, Silurian, 306. 

, on succession of types, 339. 

Nectar of plants, 92. 

, on affinities of the dugong, 414. 

Nectaries, how formed, 92. 

, on homologous organs, 435. 

Nelumbium luteum, 387. 

, on the metamorphosis of cephalo- 

Nests, variation in, 212. 

pods and spiders, 442. 

Neuter insects, 236. 

Newman, Mr., on humble-hees, 74. 


New Zealand, productions of, not per- 

fect, 201. 

Pacific Ocean, faunas of, 348. 

, naturalised products of, 337. 

Paley on no organ formed to give pain, 

, fossil birds of, 339. 

20 1 . 

, glacial action in, 373. 

Pallas on the fertility of the wild 

, crustaceans of, 376. 

stocks of domestic animals, 253. 

, algae of, 376. 

Paraguay, cattle destroyed by flies, 72. 

— — , number of plants of, 389. 

Parasites, 217. 

, flora of, 399. 

Partridge, dirt on feet, 362. 

Nicotiana, crossed varieties of, 271. 

Parts greatly developed, variable, 150. 

, certain species very sterile, 257. 

, decrees of utility of, 201. 

Noble, Mr., on fertility of Rhododen- 

Parufl major, 183. 

dron, 251 . 

Passiflora, 251. 

Nodules, phosphatic, in azoic rocks, 

Peaches in Doited States, 85. 


Pear, grafts of, 261. 





Pelargonium, flowers of, 145. 

Poison, similar effect of, on animals and 

, sterility of, 251. 

plants, 484. 

Pelvis of women, 144. 

Pollen of fir-trees, 203. 

Peloria, 145. 

Poole, Col., on striped hemionus, 163. 

Period, glacial, 365. 

Potamogeton, 387. 

Petrels, habits of, 184. 

Prestwich, Mr., on English and French 

Phasianus, fertility of hybrids, 253. 

eocene formations, 328. 

Pheasant, young, wild, 216. 

Primrose, 49. 

Philippi on tertiary species in Sicily, 

, sterility of, 247. 


Primula, varieties of, 49. 

Pictet, Prof., on groups of species sud- 

Proteolepas, 148. 

denly appearing, 302, 305. 

Proteus, 139. 

, on rate of organic change, 313. 

Psychology, future progress of, 488. 

, on continuous succession of ge- 

nera, 316. 

, on close alliance of fossils in con- 


secutive formations, 335. 

, on embryological succession, 338. 

Quagga, striped, 165. 
Quince, grafts of, 261. 

Pierce, Mr., on varieties of wolves, 91. 

Pigeons with feathered feet and skin 

between toes, 12. 


, breeds described, and origin of, 


Eabbit, disposition of young, 215. 

, breeds of, how produced, 39, 42. 

Races, domestic, characters of, 16. 

, tumbler, not being able to get 

Race-horses, Arab, 35. 

out of egg, 87. 

, English, 356. 

, reverting to blue colour, 160. 

Ramond on plants of Pyrenees, 368. 

— -, instinct of tumbling, 214. 

Ramsay, Prof., on thickness of the 

, carriers, killed by hawks, 362. 

British formations, 284. 

, young of, 445. 

, on faults, 285. 

Pistil, rudimentary, 451. 

Ratio of increase, 63. 

Plants, poisonous, not affecting certain 

Rats, supplanting each other, 76. 

coloured animals, 12. 

, acclimatisation of, 141. 

, selection applied to, 32. 

, blind in cave, 137. 

, gradual improvement of, 37. 

Rattle-snake, 201. 

not improved in barbarous coun- 

Reason and instinct, 208. 

tries, 38. 

Recapitulation, general, 459. 

destroyed by insects, 67. 

Reciprocity of crosses, 258. 

, in midst of range, have to struggle 

Record, geological, imperfect, 279. 

with other plants, 77. 

Rengger on flies destroying cattle, 72. 

, nectar of, 92. 

Reproduction, rate of, 63. 

, fleshy, on sea-shores, 132. 

Resemblance to parents in mongrels and 

, fresh-water, distribution of, 386. 

hybrids, 273. 

, low in scale, widely distributed, 

Reversion, law of inheritance, 14. 


in pigeons to blue colour, 160. 

Plumage, laws of change in sexes of 

Rhododendron, sterility of, 251. 

birds, 89. 

Richard, Prof., on Aspicarpa, 417. 

Plums in the United States, 85. 

Richardson, Sir J., on structure of 

Pointer dog, origin of, 35. 

squirrels, 180. 

, habits of, 213. 

, on fishes of the southern hemi- 

Poison not affecting certain coloured 

sphere, 376. 

animals, 12. 

Robinia, grafts of, 262. 





Rodents, blind, 137. 

Silene, fertility of crosses, 257. 

Rudimentary organs, 450. 

Silliman, Prof., on blind rat, 137. 

Rudiments important for classification, 

Skulls of young mammals, 197, 437. 


Slave-making instinct, 219. 

Smith, Col. Hamilton, on striped horses, 



, Mr. Fred., on slave-making ants, 

Sageret on grafts, 262. 


Salmons, males fighting, and hooked 

, on neuter ants, 239. 

jaws of, 88. 

, Mr., of Jordan Hill, on the de- 

Salt-water, how far injurious to seeds, 

gradation of coast-rocks, 283. 


Snap-dragon, 161. 

Saurophagus sulphuratus, 183. 

Somerville, Lord, on selection of sheep, 

Schiodte on blind insects, 138. 


Schlegel on snakes, 144. 

Sorbus, grafts of, 262. 

Sea-water, how far iujurious to seeds, 

Spaniel, King Charles's breed, 35. 


Species, polymorphic, 46. 

Sebright, Sir J., on crossed animals, 

, common, variable, 53. 


in large genera variable, 54. 

, on selection of pigeons, 31. 

, groups of, suddenly appearing, 

Sedgwick, Prof., on groups of species 

302, 306. 

suddenly appearing, 302. 

beneath Silurian formations, 306. 

Seedlings destroyed by insects, 67. 

successively appearing, 312. 

Seeds, nutriment in, 77. 

changing simultaneously through- 

, winged, 146. 

out the world, 322. 

, power of resisting salt-water, 

Spencer, Lord, on increase in size of 


cattle, 35. 

in crops and intestines of birds, 

Sphex, parasitic, 218. 


Spiders, development of, 442. 

eaten by fish, 362, 387. 

Spitz-dog crossed with fox, 268. 

in mud, 386. 

Sports in plants, 9. 

, hooked, on islands, 392. 

Sprengel, C. C, on crossing, 98. 

Selection of domestic products, 29. 

, on ray-florets, 145. 

, principle not of recent origin, 

Squirrels, gradations in structure, 180. 


Staffordshire, heath, changes in. 7_. 

, unconscious, 34. 

Stag-beetles, fighting. 88. 

, natural, 80. 

Sterility from changed conditions of 

, sexual, 87. 

life, 9. 

, natural, circumstances favourable 

of hybrids, 246. 

to, 101. 

, laws of, 254. 

Sexes, relations of, 87. 

. , causes of, 263. 

Sexual characters variable, 156. 

from unfavourable conditions, 

i selection, 87. 


Sheep, .Merino, their selection, 31. 

of certain varieties, 269. 

■ , two suit-breeds unintentionally 

St. Helena, productions of, •"•^'. , . 

produced, :;n. 

St. Hilaire, Aug., on classification, 41& 

, mountain, varieties of, 76. 

St. John, Mr., on habits of cats, 91. 

Shells, colours of, 1 32. 

stin- of bee, 202. 

, littoral, seldom embedded, 288. 

Stocks, aboriginal, of domestic animals, 

, fresh-water, dispersal of, 385. 


of Madeira, 391. 

Strata, thickness of, in Britain, 284. 

, land, distribution of, 397. 

Stripes on horses, L63. 





Structure, degrees of utility of, 201. 

Tumbler pigeons, habits of, hereditary, 

Struggle for existence, 60. 


Succession, geological, 312. 

, young of, 446. 

Succession of types in same areas, 338. 

Turkey-cock, brush of hair on breast, 

Swallow, one species supplanting an- 


other, 76. 

Turkey, naked skin on head, 197. 

Swim-bladder, 190. 

, young, wild, 216. 

System, natural, 413. 

Turnip and cabbage, analogous variations 

of, 159. 


Type, unity of, 206. 

Types, succession of, in same areas, 338. 

Tail of giraffe, 195. 

■ of aquatic animals, 196. 


, rudimentary, 454. 

Tarsi deficient, 135. 

Udders enlarged by use, 11. 

Tausch on umbelliferous flowers, 146. 

, rudimentary, 451. 

Teeth and hair correlated, 144. 

Ulex, young leaves of, 439. 

, embryonic, traces of, in birds, 

Umbelliferae, outer and inner florets of, 



— — , rudimentary, in embryonic calf, 

Unity of type, 206. 

450, 480. 

Use, effects of, under domestication, 11. 

Tegetmeier, Mr., on cells of bees, 228, 

, effects of, in a state of nature, 



Temminck on distribution aiding classi- 

Utility, how far important in the con- 

fication, 419. 

struction of each part, 199. 

Thouin on grafts, 262. 

Thrush, aquatic species of, 185. 


, mocking, of the Galapagos, 402. 

, young of, spotted, 439. 

Valenciennes on fresh-water fish, 384. 

, nest of, 243. 

Variability of mongrels and hybrids, 

Thuret, M., on crossed fuci, 258. 


Thwaites, Mr., on acclimatisation, 140. 

Variation under domestication, 7. 

Tierra del Fuego, dogs of, 215. 

caused by reproductive system 

, plants of, 374, 378. 

being affected by conditions of life, 8. 

Timber-drift, 360. 

under nature, 44. 

Time, lapse of, 282. 

, laws of, 131. 

Titmouse, 183. 

Variations appear at corresponding ages, 

Toads on islands, 393. 

14, 86. 

Tobacco, crossed varieties of, 271. 

, analogous in distinct species, 

Tomes, Mr., on the distribution of bats, 



Varieties, natural, 44. 

Transitions in varieties rare, 172. 

, struggle between, 75. 

Trees on islands belong to peculiar 

, domestic, extinction of, 111. 

orders, 392. 

■ , transitional, rarity of, 172. 

with separated sexes, 99. 

, when crossed, fertile, 267. 

Trifolium pratense, 73, 94. 

, when crossed, sterile, 269. 

incarnatum, 94. 

-, classification of, 423. 

Trigonia, 321. 

Verbascum, sterility of, 251. 

Trilobites, 306. 

, varieties of, crossed, 270. 

, sudden extinction of, 321. 

Verneuil, M. de, on the succession of 

Troglodytes, 243. 

species, 325. 

Tucutucu, blind, 137. 

Viola tricolor, 73. 





Volcanic islands, denudation of, 284. 

Wings of insects homologous with bran- 

Vulture, naked skin on head, 197. 

chiae, 191. 

, rudimentary, in insects, 451. 


Wolf crossed with dog, 214. 

of Falkland Isles, 393. 

Wading-birds, 386. 

Wollaston, Mr., on varieties of insects, 

Wallace, Mr., on origin of species, 2. 


, on law of geographical distribu- 

, on fossil varieties of land-shells 

tion, 355. 

in Madeira, 52. 

, on the Malay Archipelago, 395. 

, on colours of insects on sea-shore, 

Wasp, sting of, 202. 


Water, fresh, productions of, 383. 

, on wingless beetles, 135. 

Water-hen, 185. 

, on rarity of intermediate varie- 

Waterhouse, Mr., on Australian mar- 

ties, 176. 

supials, 116. 

, on insular insects, 389. 

, on greatly developed parts being 

, on land-shells of Madeira, natu- 

variable, 150. 

ralised, 402. 

, on the cells of bees, 225. 

Wolves, varieties of, 90. 

, on general affinities, 429. 

Woodpecker, habits of, 184. 

Water-ouzel, 185. 

, green colour of, 197. 

Watson, Mr. H. C, on range of varieties 

Woodward, Mr., on the duration of 

of British plants, 58. 

specific forms, 293. 

■ , on acclimatisation, 140. 

, on the continuous succession of 

, on flora of Azores, 363. 

genera, 316. 

, on Alpine plants, 367, 376. 

, on the succession of types, 339. 

, on rarity of intermediate varie- 

World, species changing simultaneously 

ties, 176. 

thoughout, 322. 

Weald, denudation of, 285. 

Wrens, nest of, 243. 

Web of feet in water-birds, 185. 

West Indian islands, mammals of, 395. 


Westwood on species in large genera 

being closely allied to others, 57. 

Youatt, Mr., on selection, 31. 

on the tarsi of Engidae, 157. 

, on sub-breeds of sheep, 36. 

on the antennae of hyuaenopterous 

, on rudimentary horns in young 

insects, 416. 

cattle, 454. 

Whales, fossil, 303. 

Wheat, varieties of, 113. 


White Mountains, flora of, 365. 

Wings, reduction of size, 134. 

Zebra, stripes on, 163. 


Albemarle Street, London. 
June, 1859. 


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